The magazine tube is not a user replaceable part, and it's not a good idea to allow a local gunsmith to attempt it either.
It's brazed in place with a special high-temp brazing compound.
Since the tube absolutely MUST be perfectly aligned and positioned, replacement is considered to be a factory ONLY job.
If the new tube is misaligned even slightly, the barrel will not assemble to the receiver properly.
At the factory, the receiver is totally stripped of ALL parts, including the shell releases and the entire ejector assemble.
The old tube is removed by heating the receiver until it can be removed, and the receiver cleaned of all the old braze.
A special alignment jig is attached and used to align and position the new tube absolutely perfectly with the receiver, and the entire assembly is put in a special electric furnace to re-braze the new tube in place.
After brazing, the shell releases and ejector assembly are installed and the receiver is refinished.
Due to the near impossibility of getting the new tube perfectly aligned in the receiver without the special assembly jig, and the need to totally refinish the receiver, this is best sent in to Remington to insure a quality job.
They do a perfect job and usually at as good a price as any local 'smith will.