Naturals being automatically inserted in the complete score when not required after key signature
I finished a score and then entered a D Major key signature from the
selection box. I entered this on the first bar but found that although every
line had the two sharps correctly entered very C and F had a natural against
it. Not what I expected.
Comments
Changing the key signature does not change any notes already entered. It only affects notes added after the key change.
I'm guessing you want to change the key of the song, but otherwise keep the notes the same. If there are no accidentals, the fastest way to fix this after changing the key is to select the entire score then press ctrl-up arrow then ctrl-down arrow. This will change all the notes to the current key signature. Any accidentals will be eliminated. So if you want to maintain accidentals, you will need to either make note of accidentals to fix them later, or do the previous instructions a phrase at a time to avoid accidentals. If you have any F# or C# accidentals (before the key change), they will remain as sharps and not be changed to G and D respectively as you might want.
If you want to change the entire piece to the key of D (making all Cs into Ds and so forth), use the Notes -> Transpose... dialog and tell MuseScore to transpose the entire piece to the key of D major/B minor. You will want to make sure you have nothing selected when you do this, otherwise only the selection will be transposed.
What Mike said :-), but with one minor correct - Ctrl+Up / Ctrl+Down won't change the pitch, but Ctrl+Alt+Up / Ctrl+Alt+Down will. Ctrl+Up/Down changes octave while keeping the same pitch otherwise; Ctrl+Alt+Up/Down changes pitch "diatonically" - staying within the key. So Ctrl+Alt up moves everything up a step and loses all accidentals, Ctrl+Alt+Down then moves it back down.
In the process, though, you lose *all* accidentals, not just the ones needed because of the key change. So if you had already entered a Bb, for instance, that will now become a B. Hopefully not an issue, but it isn't clear what your intent was in first entering a piece in C and then changing the key signature. That wouldn't be the usual way to enter music - you'd get the key signature correct first so that the notes you entered are already the correct pitches.
If it was just an oversight and you actually meant it to be in D all along, and there are other accidentals you entered on purpose, then you'll need a different method to fix only the F's and C's without affecting anything else. If you try a nightly build of 2.1, you could right click on C, Select / More / Same note name, then press the Up arrow button to change all C's to C#'s. Then do the same for F.
If you need further help, best to attach your score and describe what you are trying to do in mor detail, and we can offer more specific help.