Deleting!!!

See https://musescore.org/en/handbook/4/editing-notes-and-rests#deleting-no…

And see https://musescore.org/en/handbook/4/editing-notes-and-rests#remove-range

Ctrl+Delete to delete selected measures.

You may want to know how you got the measure with 5 beats in 4/4 time. At the far left of the toolbar is the button to toggle Note entry mode on and off. There are several kinds of "Note entry". The one most people use most of the time is labeled "Default (step time)" and it has a pencil as its icon. Another is labeled Insert and it has a quarter note with a plus sign (+) as its icon.

The step time entry mode works like "typeover" was selected in MS Word (or other word processors). Entering a note or rest replaces (types/ over) the existing notes/rests. The Insert entry mode adds the note before the currently selected note/rest, thus increasing the duration of the measure. I'm'a guess that you inadvertently got into that Insert mode when you were entering that 5/4 measure.

You can get rid of that extra beat in several different ways:

• Select the entire measure and press Ctrl+Delete. Then insert a single measure at the same place and re-enter correct notation. (Or vice versa)
• Select the incorrect quarter note and press Ctrl+Delete.
• Move the desired notes to the beginning of the measure. Right-click on the measure (not on any note/rest, but on blank space within the measure), click Measure properties..., and on the Measure properties dialog change the Actual duration back to "4/4".

As SteveBlower suggested, the MuseScore 4 Handbook has this information and much more.

I recommend that every new (or "new-ish" :-) user take an hour or two and simply read the entire MuseScore 4 Handbook. Don't try to memorize it! Just skim through it and familiarize yourself with the contents! But read it all!!! Then you'll have an idea where to find things when issues come up in the future. "Didn't I see something about that in the Handbook?"

Welcome aboard and best of luck!!!