No, unfortunately it is not possible to change the bitcoin address linked to your deposit code after signing up. As there are no credentials to login with bittr, we have no way to be sure that it's actually you, who would like to change the address. This would open up a security issue as people with malicious intents might convince the bittr customer support team to change your address while nothing has changed at the your end (i.e. changing the description of the recurring SEPA transfer). To securely change your address, register again to obtain a new deposit code and change the description of your recurring bank transfer.