Shamster,
You don't need to remove the magazine tube.
Of course you first start by making the gun safe. Then you...
1. Remove the barrel from the reciever.
2. Remove the bolt assembly from the reciever. You do this by holding the gun upside-down.(Sitting down on a chair with the trigger pointing up works best.) Slide the pump towards the reciever about 2 inches so the bolt unlocks and travels about 1 1/2 inches rearwards. With your thumb push the gate down and depress the thin sheet metal tab which sticks out slightly from the inside left sidewall of the reciever. This tab almost touches the follower. Once the tab is depressed you can slide the pump forearm/slide rails foward off the reciever. The manual has pictures on how to do this.
3. There is a large nut on the front end with two small slots retaining the forearm to the rails. Remove the nut and then the forearm.
4. Reassemble in reverse order.
Good luck.
Espresso