Convert notes to value?

Is there a quick way/shortcut to select a group of say 8th notes and convert them to quarter notes, or any other value?

Sure. select them and press 6 (or whatever value you want.

Make sure Insert Mode is on first! (That’s “I”)

Oh great thanks for the info! Another unrelated question - it would be handy when playing back to select a set of notes and be able to quickly mute them (not have them playback). I use Noteperformer so know I can mute from there or the mixer, but would like to do that from the Dorico Write page. Is that possible?

Yes. With the notes selected, toggle on the “Muted” property in the bottom panel and check the box.