Hi Stuart, and welcome to the forum! If you already have any notes in the bar:
- Turn on insert, the 4th mode, Global adjustment of current bar
- Insert or delete notes or rests freely
Example: select and delete a half rest to shorten the bar by 2 quarters
Dorico automatically changes the meter to fit the bar you edit, and restores the previous meter in the following bar.
When you have completely empty bars, with no notes or markings, you can just add meter changes wherever you want – but always in chronological order! because any notation you add is fixed in place in the timeline.