I cannot remove a previous payment method nor I can update it

Try adding the new card and removing the old one via https://appleid.apple.com/account/manage

If that also doesn't work contact iTunes Support and see if they can help you do it :  http://www.apple.com/support/itunes/ww  (click on your country's flag, the country where you live, and which is therefore the country on the account ; click on Contact Support towards the top right ; click on Get Help under 'Talk To Us’)