switching apple id accounts on iphone

Sign out of one and then sign into the other. You will be presented with the option to keep or delete data, so select delete unless you want data to be combined between the two accounts.

Yes, this sound complicated but iPhones are not designed for multiple user account unless you entirely erase them between users.

Read this document to learn how to sign out of iCloud on your iPhone, iPad, iPod touch, Apple TV, or Mac --> Sign out of iCloud on your iPhone, iPad, Apple TV, or Mac - Apple Support

For an Apple mobile device:

1. Go to Settings, then tap your name. 

2. Scroll down to the bottom.

3. Tap Sign Out.

4. Enter your Apple ID password and tap Turn Off. (Read this if you If you forgot your Apple ID password - Apple Support ) 

5. Turn on the data that you want to keep a copy of on your device.

6. Tap Sign Out.

7. Tap Sign Out again to confirm that you want to sign out of iCloud.

Also of possible relevance:

Turn off Find My on your iPhone or other devices - Apple Support

Read this support article to learn how to sign in with your Apple ID on various kinds of devices --> Sign in with your Apple ID - Apple Support

For an Apple Mobile device:

1. Open the Settings app.

2. Tap Sign in to your [device].

3. Enter your Apple ID and password.

4. If prompted, enter the six-digit verification code sent to your trusted device or phone number and complete sign in.

You should Loog out completely, and reset the device before changing the Apple ID on device.

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support