Are you trying to active a new SIM or change SIMs on a existing account?
If you wanted to activate a new SIM then Its one SIM card per account so you needed to logged out your current account and then followed the steps below
- Log out here https://giffgaff.com/activate
link to the activation page and enter the 6 digit activation code from the new SIM. If you lost this code, you can use the 13 or 19 digit code printed on the SIM itself (right before 3Ggg1 or 4Ggg4).
- Click "Activate your SIM", then register a new account (do not log into your existing account).
The username you enter must be unique, any other details such as email, address and payment details e.g. debit/credit card details can be the same as your current account.