Unwanted upbeat

By 'change', do you mean you 'shorten' the duration of the note, and then an 'unwanted' rest appears?

If you have already read this:
http://musescore.org/en/handbook/note-entry

You should take a look at this:
http://musescore.org/en/handbook/copy-and-paste
in order to copy the succeeding notes over the 'unwanted' rest.

If this is not the case, please attach your score (or an example of what you are trying to do). Click on
'File attachments' which displays above 'Save' and 'Preview' to attach to your forum post.
Welcome aboard.

Can you provide more information? Like, post the score you are having problems with, and then describe *exactly* - step by step, like "first click the third note of measure 6, then click the quarter note icon on the toolbar" what you are doing, what you expect to see happen, and what you see instead.

My guess is you are being surprised by the fact that if you shorten a note, MuseScore does not automatically move the subsequent notes earlier to close the gap. Instead, it adds a rest as necessary to ensure subsequent notes stay at the same time position you originally entered them in. If you want to move notes earlier in time, you need to do it yourself - select the range of notes you want to move, cut, click the place you want to move them to, paste. MuseScore doesn't do this for you because a) much of the time, after shortening a note, you wouldn't *want* subsequent notes moved, and b) it has no way of guessing how many notes you would want to move.