If it's the same as my JC Higgens 20g, then there's a screw on the side of the action you must loosen a bit (or remove completely, I don't recall).
Do some research online before you shoot it though. Some of the bolt action 12g guns were under a recall for faulty locking lugs. Folks had the bolts ejected into their faces. I'm pretty certain the recall was for the JC Higgens line, but Mossberg and a few other companies made the guns for Sears. BTW, if you have an affected Higgens bolt action 12g, I think Sears still pays a $100 bounty for the bolts.
Chris