I have a 12, it’s under my husbands Apple ID, but I can’t change it to mine. How can I change it

The above way is the best, but if you want to keep all the data, when changing to the new ID, there may be some data loss, but you can minimize it. If syncing photos with iCloud, ensure Optimization is turned off in Settings/Your Name/iCloud/Photos and/or in Photos/Preferences/iCloud. Then make sure full resolution photos are on the device/computer. When signing out of iCloud, if asked whether you want to retain data, choose yes. Messages conversations will be lost if they are addressed to the ID. See the article below for other places that the old ID should be removed. See Winston Churchill's response in the below discussion. If possible, also set up Family Sharing  between the IDs to keep access to your apps/purchases. 

 How to switch Apple IDs on your iPhone or iPad.      

How to transfer all files from one iCloud account to another.       

What to do after you change your Apple ID or password - Apple Support.      

Family Sharing.         

Family Sharing - Set up.        

Basically, your husband signs out of his Apple ID on the iPhone before giving it to you ... and you sign in with your Apple ID.

In detail, please have your husband do these steps to ensure a clean and complete handover: What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

Sign out of iCloud on your iPhone, iPad, iPod touch, Apple TV, or Mac - Apple Support

Sign in with your Apple ID - Apple Support