Contract cancellation

Hello!
I would like to cancel my contract with O2.
I already have the Abmeldung since I have to go back to my country this month.

What should I do next?

(It is a bit hard for me so I hope you can help, since there’s no option for English and my German is not good enough to understand what the menu says.) 

Also, is there any other option to confirm my contract cancellation?

TemMour