cancelled plan by mistake right after subscribing

Hi! 

I'm not good in German and had a difficult time navigating the o2 mobile application. 

I am currently using the "o2 My prepaid plan L". After topping up, I was automatically charged EUR9.99 for the "prepaid data pack L". Because I wanted to switch to another plan, I intended to select terminate the current plan after 28 days. But i must have selected the wrong option as it was cancelled immediately after i purchased. 

Is there any way to restore the plan as it was a waste of money to cancel it right away?  Is it possible if i want to change to another plan (e.g. my prepaid plan S", and how should I do that without continuing with "prepaid data pack L" automatically after topping up the account? 

Thank you very much for your help.