How to switch a Verizon Iphone 4s to Tmobile?

How do you switch a Verizon Iphone 4s to Tmobile?

It has to be unlocked (deprogrammed to work on just Verizon signals) Verizon uses CDMA and Tmobile uses GSM signals. A sim swap won't do that. You have to buy an unlock code online and they aren't cheap. Best luck.