How to erase the wrong rest or modify it

More info is needed, Upload a sample of the score with the measure and explain what you want to do.

Please remember:  

“‘Rests’ are where ‘Notes’ are not!”

AFAIK, MuseScore’s fundamental view of the musical world, with respect to each indvidual stave, is that it consists of a stream of notes, of a particular duration, at particular (absolute) moments in time.  

Therefore, necessarily, the entire interval-of-silence between these (absolutely-positioned) notes is defined by rests.

You are free to change the definition of those rests – you can, for example, combine two eighth-rests into one quarter-rest, or vice-versa – so long as “the total interval of silence defined by the totality of it” remains the same.

Rests are silence, you can't delete silence, but you can replace silence by music

Welcome to the forums.

Because MuseScore offers playback capabilities, all notes/rests have a temporal relationship to each other and even to the staff itself. In other words, one can't just move stuff around - like dragging an eighth rest from one part of a measure to another. In fact, if you click on any note/rest in a score and look at the bottom Status Bar you can see (among other things) in which measure and on what beat that particular note/rest occurs. So, changing a note/rest requires replacing it with something else.

As you wrote...
I cannot erase the rests that were there before
True... as MuseScore abhors a vacuum, a musical sound (note) must replace a rest. So, click on the errant rest and enter a note. (Since only you know what pitch belongs there, MuseScore makes no assumptions, and takes no guesses.)
Another possibility...
If you want some of the notes which succeed (or precede) the errant rest to replace it, cut and paste them onto that very rest. (Essentially a time shift of notes.)

I cannot erase... the notes for the version 2.03.
True, as MuseScore abhors a vacuum, a rest (or a different note) must replace an existing note. To delete a note(s), select it (or them) and press 'Delete' on your computer keyboard. MuseScore will automatically replace the notes with rests. (Since silence has no pitch, no guess is required.)

I urge you to read the handbook, especially:
https://musescore.org/en/handbook/note-input
and:
https://musescore.org/en/handbook/copy-and-paste

Also, the forums contain threads discussing these same basic issues. Here are a few:
https://musescore.org/en/node/101016
https://musescore.org/en/node/17010
https://musescore.org/en/node/37716

Regards.