MuseScore -- Index -- Measure Operations

Insert Measures
First select a measure then press Ins to insert a new empty measure before the selected one.
Delete Measures
Complete measures can be deleted by first selecting them with Ctrl + Click . The measure is marked with a dotted line indicating that you selected a "piece of time". Press Shift + Click to extend the selection. Pressing Del removes the selected measures.
Properties
Measure Properties dialog To edit the properties of a measure right click on an empty part of the measure and select Measure 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.