The transmission fluid on a Chevy Trailblazer is changed by placing a drain pan under the vehicle, removing the drain plug, and allowing it to empty. The drain plug is then re-installed and fluid added through the filler tube until full.
The 2002 Chevrolet Blazer came equipped in the 4.3L V6 and 4L60E 4-speed automatic transmission. The most current (as of July 2014) and downwardly compatible automatic transmission fluid specification for this application is Dexron VI.
It is a dipstick with a yellow end directly behind the oil dipstick. Pull it out, wipe it off stick it back in and check the level, be sure the engine/transmission is warm. The engine should be running to check the fluid level.