If you'd like to change a vote after you've voted but before it's received a second vote, you can change your own vote from within the Awaiting other reviewer screen. Just click on the preferred vote; your current vote shows in dark blue.
If a reference has been moved forward and you would like to move it back to be voted on again, click the "Return to _____" button below the reference. If in included/excluded studies, this will return it to full text review. If in full text review, this will return it to title/abstract screening. Note that this undoes all of the votes associated with the reference from that stage (i.e. not only one reviewer's judgements!).
The short video below demonstrates how you can reverse your voting decisions:
When screening on mobile, you can undo a vote by clicking "Undo last vote"