Signed-in users can also find this guide, with screenshots, under Help in the app.
Your permit page (the link in your permit emails) has a Your term section.
Open your permit link.
Untick Renew automatically at the end of the term to stop at the end of the term.
To leave sooner, select End my rental. If the building charges an early-cancellation fee, the amount is shown first; you pay it, then the rental ends.