deleting something

Select it and press delete will remove the element and leave silence in it's place (which is notated by a rest).
To remove time, select the range and use Tools > Remove selected range

Have a quick read through the basics covered in the online handbook; it'll be time well invested.

To delete a note. click on it and press Delete. That will leave silence behind, i.e. the note will he replaced by a rest of the same duration. As it is the same duration your measure still has the same number beats which is a Good Thing. To delete a rest, well don't. Overwrite it with a note or notes. That also results in a measure with the same (correct) number of beats. If for some special reason you want to end up with fewer beats you can use CTRL+Delete which removes notes or rests and their duration.

See https://musescore.org/en/handbook/3/note-input for more detailed information.