There are several ways to load money onto your university ID card (for paying in the Mensa/Cafe Botanik, for printing in the library ...).
Firstly, there are several machines in the canteen where you can load money onto your card using cash, card or contactless payment.
There is also the option of loading money onto your card online. You can do this via the Studierendenwerk website:
https://www.stw.uni-heidelberg.de/en/node/13.The whole process is also explained in detail there.