Add Time signature

https://musescore.org/en/handbook/time-signature-0#add-time-signature
or also https://musescore.org/en/handbook/time-signature-0#local-time-signatures

1. Hit F9 to bring up the main palette (if it's not already open).

2. Click on 'Time Signatures' in the palette to open the list.

3. Click and HOLD on the time signature you want in the palette, then DRAG it over the measure in which you want it to be placed. Hold down the mouse button until that measure turns colour, then release.

4. To change back to the original time signature, repeat the above steps in the appropriate measure.

Note that time signature changes automatically affect all staves in a score.