change Apple ID on iphone

Apparently the Apple ID on the phone is not yours. If I am correct, see Turn off Activation Lock https://support.apple.com/en-us/HT201441. 

Since you acquired your device second hand form an individual, you will need to obtain the needed password from the prior owner or have the prior owner sign out of iCloud; erase the device; and remove it from their device list. Alternatively, if the person you acquired the device from has their original purchase receipt for the device from an authorized Apple seller with the device’s serial number on it, then they may be able to persuade Apple support to remove the activation lock. Use the support options below. Alternatively, if an Apple Store is open in your area, they can go there to deal with the problem. In this case, they can make a genius-bar appointment (https://www.apple.com/retail/geniusbar/) if desired. If none of the above work, then the device is

useless to you.

Apple ID Support https://support.apple.com/apple-id

Apple Support Twitterhttps://twitter.com/AppleSupport

Apple ID account security for country-specific andregion-specific help https://support.apple.com/en-us/HT204169

Get Support: We’re here to help https://getsupport.apple.com/. Go through the options to find the best support option.

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  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.