Transposing

There is no non-free version, and it is under Tools > Transpose

In MuseScore, the word 'transpose' has different flavors.

The transpose facility can be used to transpose an entire score (or an excerpt). One example would be, say, if your singer wanted the melody to be lowered a step or two to be able to "hit the high notes".
See:
https://musescore.org/en/handbook/3/transposition
and look at 'Manual transposition' (simpler) and 'Automatic transposition' (more options).

MuseScore's transpose facility also 'automatically' transposes if, for example, you change among transposing instruments - for example, after writing a score, you change an instrument from flute to Bb clarinet. In this case, key signatures are automatically changed for the transposing instrument.
See:
https://musescore.org/en/handbook/3/transposition#transposing-instrumen…
especially take note of 'Concert pitch'.

Also, if you have an (e.g., non-standard, or even rare) instrument whose pitches 'sound' differently from what is written in the score, you can change the staff transposition:
https://musescore.org/en/handbook/3/transposition#change-staff-transpos…
An example of this would be, say, a harmonica player who can only read in C major, but switches to a 'G' harmonica (i.e., a different tuning) to play along with others reading from a score written in G major. In this case the harmonica part can be made to 'read as in C major' for the player but, during playback, 'sound as in G major' - in harmony with the other players (who are reading their parts in G major).

Regards, and welcome aboard.