how do i change the iCloud account information on my iPhone?

How do i change the iCloud Account information for an iphone. I set up my wifes iPhone and my phone with the same iCloud Account name name. Later, i added another phone on my ATT account for my mother, but made an Apple iCloud Account in her name. Now, to use the find my iphone feature, i need to change her iCloud account user name to be the same as mine. How do I change that?