How to change a contract iphone to pay as you go?

I have an old iphone which the contract has ended, how can i make this phone pay as you go?

Steps to follow:
1. You call your mobile network and ask to terminate the contract
2. They'll tell you the date when it is getting terminated
3. Day before the date get PAYG SIM from any network
4. Get PAC code from your previous network
5. Call your new network and give them your PAC code (to have same phone number on PAYG)
6. Off you go

Note: If you're going to use some other network then don't forget to unlock your phone

Your carrier may unlock the iPhone for you. Call and ask, since your contract has ended they usually are pretty cool about unlocking your phone.

After you unlock it you can take any prepaid SIM and just use it with that unlocked iPhone.