Look on the back end of your scope, away from the eyepiece and down where the main mirror is located. The collimation screws are paired with locking screws (which act to hold the mirror in place to maintain collimation) for a total of six screws.
The most clockwise screw of each pair (as seen from behind the mirror) is the collimation screw. You can verify this by looking into the tube from the front. The exposed screws around the mirror cell will turn as you turn them from behind the mirror with a screwdriver.
Always loosen the locking screws first before turning any of the collimation screws. You may need to back out the collimation screws if turning one doesn’t give you enough travel to properly center the image of the scope’s secondary mirror.
When finished adjusting the collimation screws, tighten the locking screws to hold the mirror in place.