Bremen tram company website in English. Not all of it works (I can't get the maps) and the layout is a bit hit and miss but it looks like the trams have ticket machines on board - I'd guess they also have them at the Airport stop. A single ticket to the centre is €2.10.

If you meant can you offer the driver a €50 note and get change, the answer is probably no!

Thanks for the link. No, I was thinking of giving change!