It's best advised to change the fluid with the tube out of the set. Some guys in the shop like to do it on-board but personally I can do it just as fast by pulling the tube. Removing the tube is no big deal, after removing the front screens and back cover you'll have to pull the crt socket board off. This is probably the hardest part since Philips puts a glob of RTV between the socket and the tube, as you're pulling down, gently wiggle the board, it will pop off. You'll have to pull the HV anode lead out of the HV block on the LSB board in the back. Unplug the convergence plug for the particular tube you're changing as well as the Yoke plug. There are normally 2 ground wires connected to the backside of the CRT metal shielding. Those have to come out. After removing the 4 1/4" screws holding the crt down, slowly remove the CRT. BE very careful that you don't bang or bump the certering rings when you remove the tube. Those are a pair of rings around the neck of the crt with tabs on them. They're usually RTV'd down but they can be moved. If you move them you'll have to recenter the tube.
You'll find the plug on the top of the coolant chamber, twisting 180 degrees loosens it so you can remove it. Careful, there's a small white washer you must keep and it tends to fall and can get lost. Drain your coolant into a container like a kitty litter container. After the fluid is drained, removed the 4 screws holding the lens on and clean the inside of both the lens and CRT coolant chamber with a rag. Reinstall the lens. Sit the tube upright, begin slowly filling the chamber back up with the replacement fluid until it reaches the very top. Reinstall the cap with rubber gasket washer and tighten 180 degrees. Clean any coolant on the outside of the tube off, you don't want any of this stuff dripping down into the set.
Reinstall tube the same way you removed it. As long as you didn't move those centering rings, any convergence adj. should be very minor. Proceed to do the same to the rest of the tubes.
Normally the blue tube dirties the worst and fastest. The glycol based coolant is extremely sticky and corrosive as well as conductive. If you do the coolant change in the set and you spill any coolant on the PC boards, STOP WHAT YOU'RE DOING AND CLEAN UP THE SPILL IMMEDIATELY! AS I said, this stuff is conductive and will short circuit the boards it's spilled on quickly.
If you're not comfortable doing this call a pro.