The procedure for Adjusting belt tension is not particularly hard, but does require a specific sequence of steps. The only tools required are a 15mm box/open ended wrench (or adjustable wrench), a 6mm allen wrench, and 3 or 4mm allen wrench. The early versions of the bicycle used 3mm and the recent version uses a 4mm.
You can download an app for Android phones to tension it according to sound, but I haven't found the app to work very well. I prefer to just do it by feel, looking for about 1/8" to 3/16" deflection at the belt.
You can download the app there, and if you use it, you want to aim for a value between 35-50 Hz.
Gates also manufactures a belt tension tool called the Gates Krikit which can be purchased through your local bike shop
First loosen the axle nuts and verify that the wheel axle is all the way in the top of the drop out slot. This is done by resting the bicycle with wheels on the ground (perpendicular to the ground as if you were riding) and pushing down on the rear rack before tightening the axle nut to secure the wheel. If the wheel is not butted up against the top of the drop out slot, the belt tension will change every time the wheel is removed for service.
If the axle slid upwards during the above procedure, then you may actually be done at this point, assuming that the belt tension reads within the proper specs.
If the tension is not right after seating the axle properly the procedure is as follows (refer to picture at the bottom):
1. Loosen sliding drop out bolt A and B on both sides of bicycle
2. Starting with drive side/right side (side belt is on) turn the tension bolt until belt has achieved proper tension. Use your preferred method to check tension-app/krikit/deflection amount.
3. Tighten the non drive side/left side tension bolt until wheel is centered in frame-double check tension to verify it is still correct
4. Tighten down sliding drop out bolt B on both sides of the bike
5. Loosen the tension bolt slightly on both sides of bike-2-3 turns
6. Tighten down the sliding drop out bolt A on both sides of bike
7. Tighten down the tension bolt on both sides