STUCK in a Key

If you enter notes in one key then MuseScore assumes that you want "normal" notes for that key - A's placed in a key of Ab will, for instance, be Ab and the B's will be Bb etc. If you change the Key Signature after entering these notes (to G Major, for instance) then any new note entered will assume G Major and you'll get A, B, E naturals and F's will be sharpened BUT any note you have already entered will not change their pitch and so you will see lots of flats.

If this doesn't explain your situation, can you post the score, please?

You probably want to transpose your score from A major to G major.
Here is where this is described in the handbook:
https://musescore.org/en/handbook/transposition

To clarify:

Changing the key signature does that and that only - it does not change any notes you have already entered. If you want to change the notes you have entered, you need to use one of the transposition functions. Depending on what you want transposed and why, selecting a region and using Notes / Transpose might be the best thing to do, or selecting and using the up & down arrows, or simply pressing F2 or Shift+F2, or changing the instrument associated with a staff to one that uses a different transposition, etc.