SAFETY FIRST - Lasers are dangerous, I'm assuming you know this and you know what you're doing with lasers and soldering irons. Never run a laser tube outside of the laser-chassis, never defeat any safety systems built into your laser cutter!
The CO2 laser tubes found in many laser cutters/engravers only have a lifetime of around 1000 - 2000 hours, after which they need to be replaced.
While replacing the tube for the BuildBrighton Hackspace laser, I thought I'd document the process as there's not a huge amount of info out there!
Craft knife (for cutting silicon tube)
Soldering Iron & Solder
Screw drivers (for releasing laser tube clips, exact driver depends on laser).
Step 1: Detach old tube
The laser tube will be soldered on at each end. The points where the wire is soldered may be covered by insulation tape, silicon sealant, heat-shrink etc. Ours was covered by silicon tubing & sealant.
Cut back the covering until you can see the wire & solder. Heat the solder with a soldering iron while pulling gently at the wire, it should pop off. Repeat for both ends of the tube.
Pull the water-supply pipes from each end of the old laser tube, if they are very tight you can chop them off as long as you've got enough slack in the pipe to make it to your water container still.
There will be a couple of clips holding the tube into the laser-chassis, undo the clips and lift out the laser tube.