Changing the prepaid from S to M

Hello!

When I started to use my sim Card M Tarif was activated and I was using that one. Then I cancelled it and started to use S Tarif but now I want to Change it again but I’m not able to do it from the Website. I did the earlier Change on the Website but this one it tells me to contact the costumer Service. How can I solve this Problem because at the Moment I don’t have any Tarife. Should I go to any O2 shop to solve it?

I appreciate your help.

Meltem

Edit by MXP_Alex: Moved to English o2 Community