How do I delete one individual note?

You can't delete rests in the first voice, you can only make them invincible.

filled with notes and/or rests. You can select and cut or copy whatever you have after the rest you don't want, and paste it over the rest in question. Then take a look at the far end of the pasted material and see if there's anything that needs fixing. MuseScore always overwrites, it never inserts, except that you can insert new blank measures.

-- J.S.

Here's another way of thinking about it: there is no such thing as "deleting" a rest in music. You can't delete silence. What you presumably want to do is replace that silence with something else - such as, the sound that would otherwise have come next. So that's the key to making it happen - enter what you want to hear instead of the silence represented by the rest and the rest goes away by itself. If the sounds you want to hear are already entered but in the wrong place, copy and paste will do the job.