Termination of my O2 Free contract due to moving abroad

I want to move back to my hometown (Umzug ins Ausland). I have de-registered my place of residence (die Abmeldebestätigung von Bürgerbüro). My flight will be in next 3 days. How can I end my O2 Free contract as soon as possible?

How much do I have to pay in extra to terminate my contract?

Thank you so much

Best regards,

Edit o2_Vivian: Mobilfunkvertrag → o2 english community: mobile