How to split a measure

In MuseScore 1.3:

1. Select the measure
2. Press Ins to add a new one
3. Right-click into the new measure → measure properties → change actual duration
4. Select the old measure again, cut (Ctrl+X)
5. Select the new, paste (Ctrl+V)
6. Right-click into the old measure → measure properties → change actual duration and tick 'do not count'
7. Optionally change the barline to a dotted one (drag from the barlines palette), or make it invisible (right-click → make invisible)

In MuseScore 2 it is more simple and a specific handbook page can be found here: Measure operations: Split and join
In MuseScore 3 it is even easier (resp. an additional method is available), see Measure operations: Split and join