Need to know how to delete rests from music

Overwrite them: https://musescore.org/en/handbook/note-input

Depends on why and where exactly you want to remove them.
You can delete notes by selecting them and pressing Del, that replacers it with a rest.
You can delete rests in voice 2-4 by selecting them and pressing Del.
Voice 1 rests though cannot get deleted, you can't remove silence except by replacing them with noise

Assuming you didn't use the special "insert mode" that is designed to add extra beats to your measure, then your measure presumably has the correct number of beats already. So you don't technically want to delete any beats, just replace sound with silence or vice versa. Normally, it's done exactly like that - by replacing what you don't want with what you do want instead.

To replace silence (a rest) with sound (a note), simply move the cursor to the rest and enter the note you want instead. To replace sound (a note) with silence (a rest), simply move the cursor to the note and enter the rest you want instead.

There are other things you can do also, like use cut & paste to move a passage of music you accidentally entered a beat early (or a beat late) to the proper time position. Thi should get you started but if you have further questions, just attach your score and explain more detail what mistake you want to correct, and we can assist better.