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!).
When screening on mobile, you can undo a vote by clicking "Undo last vote"