MuseScore -- Index -- Measure Operations

Insert Measures
First select a measure then press <Ins> to insert a new empty measure before the selected on.
Delete Measures
Complete measures can be deletet by first selecting them with <Ctrl+Click>. The measure is marked with a dottet line indicating that you selected a "piece of time". Press <Shift+Click> to extent the selection. Pressing <Del> removes the selected measures.
Properties
Measure Duration
Normally the nominal and actual duration of a measure is identical. An upbeat can have a different actual duration.

Irregular
An "irregular" measure will not be counted. Normally an upbeat is not counted as a regular measure.

Repeat Count
If the measure is the end of a repeat you can define how often the repeat should be played.