A PAC code (Porting Authorising Code) lets you move your mobile number from one network to another.
The only way you can get your PAC code is from your network provider. Here's some contact details for the main ones.
When you've got your PAC code, you'll need to give it to your new provider within 30 days, otherwise it'll expire and you'll need a new one.
Before you move your number over, make sure that you've backed up any contacts that are saved on your old SIM. This is because that SIM will be deactivated, and you won't be able to use it anymore.
Hopefully that's you sorted, but if you need more help, you can get in touch with a Team Knowhow Expert here.