How to Remove Existing iCloud Account & Switch to New/Different iCloud Account

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Removing the Existing iCloud Account from your iOS Device
*You will need physical & password access to the device*
First you’ll need to remove the existing iCloud account that is in use on the iOS device:
1.     Open the Settings app and go to “iCloud”
2.     Scroll down under all the settings to find “Delete Account” (or “Sign Out”) and tap on that
3.     Confirm the removal of the iCloud account from the device by tapping on “Sign Out” or “Delete”  
Note that iOS 9 (and up) versions of iOS use “Sign Out” in the iCloud settings panel, whereas the prior versions use “Delete Account” – the effect is identical, only the wording is different. Both will log out of the iCloud ID account on the iPhone or iPad.
*Note this removes all documents that are from iCloud from the phone or iPad (i.e. iCloud Photo Library), but not from iCloud itself. Whether or not you want to save contacts and calendar data is up to you.
You will receive the following prompts:
"If you sign out of your account, all photos in iCloud Photo Library and iCloud Drive documents and data stored in iCloud will be removed from this iPhone"
“Are you sure you want to continue? All iCloud notes will be removed from your iPhone”
“What would you like to do with iCloud Safari data, News data, Reminders and Contacts on your iPhone when the account is deleted?”
Once the iCloud account has been removed from the device, you’re left with a blank iCloud login. Here you can either create a new Apple ID and accompanying iCloud account, or change to another iCloud account.
Switching Device to a New/Different iCloud Account
This effectively lets you change between iCloud accounts on any iOS device.
1.     Follow the above steps to remove the existing iCloud account from the iOS device
2.     Enter the new / different iCloud Account credentials and log in as usual by tapping “Sign In”
3.     Choose the iCloud settings to use with the new account ID
That’s it, the iCloud account associated with the iOS device has been switched.
While this can remove iCloud and all related services from a device, this is not a replacement for resetting an iPhone to factory settings, which completely clears out all data and essentially performs a fresh iOS installation. A factory reset is NOT necessary if you simply need to change the iCloud login.