Badger:

Thumbshifters ARE GONNA BE OLD -- obsolete tech. TRIGGERS are the thing now.

But, for any of them, it's pretty much the same:

1. Make sure your new shifters match the number of gears in your drivetrain. (6, 7, 8, 9)
2. Remove the grips. This can be done with compressed air, or with an old wheel spoke and some Windex; work the spoke under the grip, pry it up a little, spray in the Windex, work the spoke around inside the grip (spreads the Windex), and pull the grip off.
3. Using the RIGHT TOOL (NEVER use standard allen wrenches where you need metrics!), loosen the old shifter and slide it off the bar. If your new shifters come with cables, you can cut the old cable anywhere it's exposed; but you must eventually loosen the cable from the derailleur.
4. Slide the new shifter on; do not tighten yet. Make sure the inside of the grip is dry, hit it with a little spray paint, and slide it back on the bar. THEN you can properly position the shifter.

The first time, it can be intimidating; but, I've done it a hundred times (literally, I build bikes for a living), and can almost do it in my sleep. It's not brain surgery.