How do I change the Apple ID on one of my devices?

If you want to change your Apple ID or your Apple ID password, see:

Change your Apple ID (Be very aware of the restrictions here.)

https://support.apple.com/en-us/HT202667

Change your Apple ID password

https://support.apple.com/en-us/HT201355

 What to do [before or] after you change your Apple ID or password

https://support.apple.com/en-us/HT204071 

If you want to use the same ID on both devices, see below. If that isn't what you want, please provide more information about what you want to do.

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. Also set up Family Sharing to keep access to your apps/purchases.

How to switch Apple IDs on your iPhone or iPad. https://www.imore.com/how-change-apple-id-your-iphone-or-ipad

How to transfer all files from one iCloud account to another. https://discussions.apple.com/thread/7452747

What to do after you change your Apple ID or password - Apple Support. https://support.apple.com/en-us/HT204071

Family Sharing. https://support.apple.com/en-us/HT201060

Family Sharing - Set up. https://support.apple.com/en-us/HT201088

You are welcome.