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!

Tools required:
   Craft knife (for cutting silicon tube)
   Soldering Iron & Solder
   Screw drivers (for releasing laser tube clips, exact driver depends on laser).
   Wire strippers
   Wire cutters

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.

Step 2: Prepare New Tube

Your new replacement laser tube should have two stud-like posts protruding upwards at each end.
To aid soldering onto these, find some multi-stranded or flexible wire, around 15cm/4 inches should do, then strip the shielding off.
We'll use a strand of the wire at each end to wrap around the post to help the solder adhere to the post when pre-soldering them. Leave a couple of inches of wire un-wrapped, we'll use this to tie the leads onto the tube.
Solder the wire onto the post at each end, making sure the solder covers the wrapped wire, but leaves the spare wire un-covered.

Step 3: Install the New Tube

With the tube to the rear of the laser-chassis, push the silicon water-supply pipes onto the inlets at each end of the laser tube. You might find it easier to achieve this if you warm the pipes up in some warm water.

You're now ready to install the new tube. Place it carefully in the clips within the laser chassis & tighten them. Pull any spare water pipe through the back of the laser & out of the way.

Now it's time to solder the supply-leads onto the tube.

Before attaching the supply-leads, cut a 2cm length of silicon tubing for each end and place over the supply-lead. We'll use this later to cover the bare soldered connection.

Take the supply-lead and tie it onto the solder-covered post protruding from the laser tube. Simply wrap the spare wire coming out of the solder around both the post & the supply-lead. Now solder the lead to the post, making sure you cover all the wire that's tying them together.

Once the connections are soldered, push the small lengths of silicon tubing over the connections for safety.

Step 4: Test!

You can now try the new tube out, connect up your water pump, run it and check for leaks first.

Your laser will be completely out of alignment, so make sure you align all your mirrors before trying a high-powered test!