What do y omena by sign in?
To change the signed in ID go to Settings>iCloud and scroll to the bottom of the back and tap delete, You will be prompted to enter the password for the singed in account.
An iCloud account is just an Apple ID. to change the password:
How do I change or recover a forgotten Apple ID Password?
If you've forgotten your Apple ID Password or want to change it, go to My Apple ID and follow the instructions. SeeChanging your Apple ID password if you'd like more information.