You need a special tool to remove the mag tube from a Mossberg. Brownells sells them for around $35 plus shipping. You also need the cap and pins. The total will be around $50. You will likely come out cheaper getting a gunsmith to fix it for you.
Do they actually make mag tube extensions for mossbergs? I own 2 the 535 and mavrick 88Dude you don't have to unscrew the tube to get access to the spring. When you take off the cap in the front and remove the barrel you will notice in the from of the tube there seems to be a cap that looks like it won't come out because the end is crimped get a large flat heat stick it in that cap and screw in about 1/8 of a turn do that the notch in the cap lines with the crimp in the tube and pops out mind you, you need to do this pushing the cap in a little. Once the cap comes off just pull the spring out and follower. If you feel the spring has lost some tension give it an attitude adjustment and stretch it out a bit to gain more tension. You don't need any special tools. The only reason to take out the mag tube is if it's bent or dented because it can cause feeding or loading issues.
They make magazine extensions for a mossberg but you have to use an aftermarket barrel. Brownells sells the kits. Your gun looks weird with a magazine tube that is a foot longer than the barrel.Do they actually make mag tube extensions for mossbergs? I own 2 the 535 and mavrick 88