I can't get into the iPhone

my xemployee wiped out my company cell phone and now I can't get into the phone to have a new employee use it and activate a new phone number with AT&T. I purchased this phone on 3/10/2016 est on 3/10/16 Apple Iphone6Plus for 849.99 and now I want to give it to another employee but when this employee left the company he wiped the phone out and I can't get the new password he put in it and need to know how I can reset this phone so I can use it for a phone number with have for the new employee. He used itune user name. I would like to change that.

thanks,

Kim S.

[Edited by Moderator]