remove sim, clean and wipe then replace and restart your phone
order another sim here https://giffgaff.com/orders/mgm
you may need to back up your contacts from your old sim first
For sim swap, log into here https://giffgaff.com/activate
Put the 6 digit code in from your new sim, And click continue,
On the next page it will show the 6 digit activation codes and sim serial numbers of your current and new sim cards.
Click where it says... "Yes I want to replace my SIM"
On the next page click "yes i am sure"
swap will not take long, and you will be up and running with your new sim/credit/goodybag and current number in no time at all