Transposing for Tuba

There's a bunch of Tubas MuseScore offers, 5 transpose, 8 don't

See: https://musescore.org/en/handbook/3/accidentals#change-enharmonic-spell…

It's still not really clear what you mean here. Music can have both flats and sharps, and tuba is not different from other music in this respect. If the music has sharps when written for other instruments, it should have sharps for tuba as well. It would be totally wrong to make music that currently has sharps and then simply respell those sharps to flats, if those sharps were correct to begin with.

Now, it's possible you are saying you want the music in a different key - a key that is higher or lower than the original key, and that this different key might happen to have flats in its key signature. If so, simply use Tools / Transpose to transpose the music to that new key. It's still the case that accidentals in the key can sometimes be flats, sometimes sharps (and sometimes naturals) - tuba is not exempt from the rules of music notation. In the key of G minor, for instance, there are two flats in the key signature, but the leading tone is absolutely always spelled F#, never Gb.

If you continue to need help, please attach your score and describe what you are trying to do in more specific detail.

Here's a simplified method that should work to correct all sharps to flats after you've copied the line into the bass clef staff.
Simply select the entire tuba line (beginning to end) and raise it 1/2 step with the up arrow. Then lower it back down 1/2 step with the down arrow.
Example. C# is raised to a D, then the D is lowered to Db = desired result

I guess I'm not understanding the problem here. Create a tuba staff. C+P the melody into it. Ctrl + down arrow to put it in the octave you want. Since there aren't that many sharps in the melody, manually change each one. Maybe some variation of " Select similar" from the staff properties window might help. Bb tuba isn't a transposing instrument, as such. Fingerings take care of it.

As to never using sharps in student tuba music. Let's think about this. Perhaps some publishers do this, but ask yourself, why. Sharps are hard to read? Really? And why just tuba? Don't trumpet players have the same problem. As a trumpet player with a music education degree in the USA, I can tell you this is news to me. Oh sure, we know that there is the belief that brass players might prefer to read flats instead of sharps. So what. Changing sharps to flats just for tuba players is illogical, and does the players a disservice.

Thanks for the further clarification.

You wrote:
If someone could describe the steps I should take to transpose ONLY the melody line to student level bass clef melody (using only 1 to 4 flats in key signature...

Your attached midi is in the key of C major. There is no need to use any flats in the key signature, as tuba is non-transposing.
Also, I have attached this file:
Baby Face.mscz
which is a MuseScore (.mscz) file. (Simplified for students - the melody does not have those complicated note durations which result from the midi import.).
Please use this .mscz file and explain what key (how many flats in the key signature) you desire, if transposing the key.
Also, if you are only concerned with shifting notes by an octave (and retaining the C major key signature), instead of using transposition, you can press Ctrl + A to select all, then press Ctrl + up or down arrow to change the octave.

FWIW, "BBb" tuba means the fundamental tone is the Bb an octave below that of other low Bb instruments like trombone.

The reason we needed more information is that the correect way to achieve the goal of fewer sharps depends on the context. We simply didn't know if the problem was the accidentals were wrong to begin with, or the kley was wrong, or they were correct and you simply wanted to change them to be incorrect. So thank you for providing the file you are asking about, it helps clarify.

I need to reiterate: it is simply not true that student music is never written with sharps. As I said, when sharps are musically necessary, they are absolutely written, ebcause writing music incorrectly is the surest way to confuse young students (and it does no favors for more experienced ones either). What is true is that beginning band music is most often written in flat keys, because that makes it easier for transposing instruments like saxophones. But as I said, even in flat keys, sharps will still appear where necessary. So that's why we needed to understand the context for the sharps you were asking about - to understand if it's correct to simply change their spelling, or if it would be necessary to change the key, to make it correct.

So, now that we have the file, we can see that your piece is in the key of C. Most of the sharps that appear are absolutely correct musically and should not be changed - if you leave the song in the key of C. But if the goal is to create music that is easier for very young students, you should transpose it to a a key like Bb. So, Ctrl+A to select all, then Tools / Transpose, then select the key of Bb or whatever other key you prefer. This will both transpose and respell everything to be correct - as correct as it was in the original anyhow. Unfortunately. MIDI is not a notation format and lacks information about spelling, so it requires AI algorithms within MuseScore to tyr to guess at the right spellings. It guessed right in almost all cases here, but indeed, that means some flats, some sharps. When you transpose to Bb, it will be almost all flats.

The bigger prolblem you will have also stems from this being a MIDI file - the rhythms are almost completely unreadable, because theis was nevcer intended to be converted to standard notation. So you're basically going to need to rewrite the melody entirely to make the rhythms simple enough to read.