Okay. I have a 12 gauge mossberg 500, so the procedure may be slightly different.
first, make sure the gun is completely unloaded.
then, slide the pump handle about half way to the rear. (this is important for clearance reasons, the pump handle will interfere with the barrel if you try to take it off with the pump in the rearwardmost position. (if the pump is forward, you won't be able to take it off because the locking lug will be locked into the barrel extention.
now, there should be a nurled knob on the end of the magazine tube. loosen it (it's got a detent on the underside so it'll click untill for the first two turns) untill the barrel slides forward off the magazine tube and the screw is completely free of it's threads.
continue pulling the barrel straight out untill the extention comes out of the reciever.
assembly is in the reverse order.
When you replace the barrel, make sure it's only hand tight. ( i go one click past the part where everything mates up, but you don't have to.