2 different apple ID's

Sign out of everything in the device with the AppleID you don’t want to use (so iCloud, messaging, FaceTime, the App, music and book stores). Then sign in with the AppleID you do want to use.

If you don’t want the iCloud data stored on that device to merge with your other AppleID’s iCloud data, then erase the device first and restore it with a backup of the other device.

Adding to the above, if you want to keep the data on the iPad, 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.