How to delete measures

1. Click in a empty part of the first measure you want to delete, or the first note.
2. Press Shift⇑ and keep it held.
3. Click on the last measure you want to delete, or the last note of this measure.
4. Release Shift⇑.
5. A blue frame should appear around the measure you want to delete.
6. Go to Edit→Delete Selected Measures (MuseScore 1 and 2) respectively Tools→Remove Selected Range (MuseScore 3), or press Ctrl+Del (Mac: ⌘+Backspace).

If you just want to delete the content of the measures, press Del (Mac: Fn+Backspace).