Preload is adjusted by a hole in the side of the shock collar. You insert a suitable screwdriver, even the allen key supplied for seat removal, or similar implement of around 6mm diameter and turn.
Clockwise to increase pre-load. I mean clockwise as viewed from the underside of the shock
The owners handbook has a comprehensive instructions under "rear suspension adjustment" pages 83, 84.
Triumph do make a tool, sold for around $3.00 but as you can see it's just a piece of steel rod: