A friend asked me this question - I don’t know the answer and couldn’t find it in the documentation (typed “change duration of pickup bar” in the search box and got over 2000 results. Gave up after the first half-dozen screenfuls. Not helpful).
See screenshot. Pickup is 2 beats but should be 1.5 beats. I double clicked the signpost and saw: 4/4,2. Changing it to 1.5 got rid of the extra half-beat but also rebarred the entire piece. Surely there’s an easy way to do this?
Activate Insert Mode - Global Adjustment of Current Bar[1] (the 4th sub mode).
Select the note or the rest that you want to remove from the pickup bar.
Press Delete.
Switch off Insert Mode
Edit : Since it looks like you’ve removed the rests at the end of the bar, first select the E in the top staff and the F in the bottom staff and switch off Ends Voice in the Properties Panel: your eighth rests should then reappear.