Not sure if it is on Apple, but android has Berlin Subway BVG Map & Route, made by Mapway. Works offline, you put in where you are (or let phone plug in location), put in destination and it tells you where to go.
App shows a fast and a simple route. In this case, both same time, single line but two options. First says S75 to Westkreuz, second says S5 Spandau and get off at Westkreuz. Either come back how you came, or get reception to call/bundle into an available taxi outside. Depends what time you finish, I think S Bahn finishes about 1am? Might be different at weekends.
To get there, go all the way to the top of Westkreuz station, turn left past a blue and yellow tyre place and keep going. If you miss it after that you are probably hammered.
