how to delete notes

By replacing them with notes, normally

Essentially you don't.
MuseScore doesn't know (and actually dooesn't need to) the notion of "empty" measure.
Each measure is always "correctly" (according to the time signature) filled in, either by notes or by rests.
And for new scores or new measures, by rests obviously, not by notes.
Conclusion : just write your notes where you want them, they will replace the rests.

Both the answer above are correct, as are other possible answers. Like, "for rests entered into voices other 1, simply select them and press Delete", or "if the goal is to move some notes earlier, just cut and paste them - no need to delete the rest at all" or "if you are entering entered music for tenor & bass in voices 3 & 4, don't, that leaves rests in voice 1 - just enter into voices 1 & 2", or " "if the desired result is a measure with fewer beats than the time signature specifies, select the rests and press Ctrl+Delete to complete remove that slice of time from the score".

In order to assist better, we would need to understand why you are trying to delete rests in the first place - it's not a common thing to need to do. So feel free to attach your score and explain the actual goal.