Hello @Akhilesh, welcome at our Community. Nice to have you here.

A prepaid card doesn't really have to be canceled because it deactivates itself after a certain period of time if you don't top it up and don't use it.

However, you can send your cancellation to the following address:

Telefónica Germany GmbH & Co. OHG
Kundenbetreuung 
90345 Nürnberg

Kind regards, Ines.