Beginner drum notation question

Try to watch the tutorials of the new version https://musescore.org/en/tutorials
HTH

It's often easier to simply re-enter the rhythm correctly rather than try to coerce an incorrect rhythm into a correct one. You say you are unable to replace the rest, but actually that is very easy. Just click it, go to note input mode, then enter your notes and rest. So:

1) click the quarter rest
2) N
3) 4 0 B B (or equivalent mouse clicks) to enter the eighth rest and two eighth notes

That said, it is certainly possiuble to correct errors like this without just re-entering the incorrect notes and rest. The most basic tool to use in changing rhythm is cut and paste. That is, cut the note from where it is (Ctrl+X), then paste it where you want it (Ctrl+V).

So, if you've entered bass drum on "1 3 3+" but you really want it on "1 2+ 3", to fix this, you could cut the note on 3+ and paste it to 2+. First you'll need to enter an eighth rest on 2, otherwise there is nowhere to paste for 2+.

Complete steps are thus:

1) click quarter rest on 2
2) press "4" or use toolbar to turn it to an eighth rest
3) click bass drum note on 3+
4) Ctrl+X to cut
5) click eighth test on 2+
6) Ctrl+V to paste

Probably you'll also want to turn the drum note on 3 into a quarter.