change time signature

Move the 6/8 time signature from the palette (on the left side) above the existing 3/4 time signature:

https://musescore.org/en/handbook/time-signature

(see the part of replacing time signature)

Yes, you can. Open the 'Palettes' list by typing F9, then click on the Time Signatures menu. Click and hold on the icon for 6:8 time...

... and drag it over to the first measure of the score, holding down the mouse button until that measure turns blue-grey. Release the mouse button, and you're done.

Note, however, that this will simply change the time signature, it won't rewrite the rhythms. So if you were hoping that rhythms of the form quarter-quarter-quarter in 3/4 will turn into eighth-eighth-eighth in 6/8 (which is how that rhyhtm would normally be written), you will be disapponted - I'm afriad you *will* have to rewrite it.

(Perhaps ... “to clarify ...”)

MuseScore is aware of exactly where the notes that you have written “exist, in the continuum of time.”   (In other words, “digital computer that it actually is,” it knows how many (partial) seconds have elapsed between “this-note” and “that-one,” at whatever-is the current tempo.)

And so, if you, for example, “change the time signature” for a particular passage, then MuseScore will dutifully and unerringly recalculate the now-correct visual expression of “that timing,” based on the new signature that you have stated, but (by design ...) it will not consider “that timing” to have changed.  In other words, It will not affect the notes themselves, nor their actual position in time.   Instead, it will recalculate the notation accordingly, so that “a human player, faced with this notation,” will play the same notes with the same timing as before.

Since 6/8 is functionally identical to 3/4, MuseScore should of course do the change perfectly.