I have included an image for reference. There is a tensioner included in the pulleys the belt travels around. (Red X). There is also a spring loaded lever point you use to release the tension on the belt.
Remove the covers to expose the belt. **Take a photo or draw the route the belt takes so you don't forget when putting the new one back on.**
Facing the belt and using a ring spanner or swan neck spanner, turn the adjuster clockwise, (green circle). **NOTE** There will be a lot of tension on the spring so take your time as it could slip and snap back into place, taking at least one knuckle with it. I always find a helper at this point makes the whole thing A LOT easier.
Once the adjuster is swung over the belt should slide off one of the pulleys and then off completely. Gently ease the adjuster back to it's rest position.
Replace as follows. Using your reference image and your helper, swing the adjuster clockwise as far as you can. Position the belt, making sure it is squarely positioned on each pulley. GENTLY ease the adjuster back off and recheck it is squarely set on all the pulleys.
At this point you can start the vehicle to check the belt in running correctly then after turning the engine OFF refit any covers over the belt.