log in , then my plan, plan features, and you should see options that you can expand 'add services' and there should be the option to tick a box for 'remove content lock' within the options (click the arrow on each to drop down the options)
EDIT so sorry ignore that; log in to my tmobile and 'Click on My Phone on the left hand menu.
from the dropdown box at Content Lock select "remove" then press "set"