How do I switch to a different Apple ID without losing contacts or data from the phone ?

Switchingusernames wrote:

I had used my wifes Apple ID for my phone but have created my own Apple ID. How do I make sure I don't lose any of my contacts or data that is on my phone?

Here's a nice guide on how to do just that:

How to “unshare” an Apple ID - Apple Community

Make a manually backup , log out from AppStore , FaceTime , iMessage then iCloud , in that screen you will see few options that you must enable all the wait for few minutes and log in with your new Apple 🆔 and tap over the alert in the MERGE button , plug in the charger and all your data it’s going to synced with your new Apple 🆔