Hi,
You need to master reset your phone.
Here is a link that shows how to do it.
''Be aware that a master reset will erase all downloaded apps and user data. It will restore the phone to a factory default condition. It will be like when you first bought it.''
'''Notes:'''
Ensure that your battery is charged.
Do not perform a master reset with the charger connected.
Remove SD Card if installed before performing a master reset.
https://support.t-mobile.com/docs/DOC-9478