How can i add a mobile phone contract to an existing contract?

I bought a contract from O2 two years ago when i came to Germany and also a Note 8. Now my contract is ending and i want to add another phone to my contract. How do i do that?

More details:
I bought a 10GB O2 M contract with a Note 8 vertrag as well from O2. Now both those contracts ended, but i would like to stay with O2 and i want a new phone, but want to add it to my contract to get the phone discount, otherwise i will cancel my contract, because its too expensive and im a student.

Edit by o2_Tatjana: moved thread to an other category