its simple , First if you haven't reset you Iphone you should do this
if you won't here is how can you remove your icloud id
- Open the Settings app and go to “iCloud”
- Scroll down under all the settings to find “Delete Account” (or “Sign Out”) and tap on that
- Confirm the removal of the iCloud account from the device by tapping on “Delete” or “Sign Out
Note : this removes all documents that are from iCloud from the phone or iPad, but not from iCloud itself. Whether or not you want to save contacts and calendar data is up to you.
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.