GMC 2010 Sierra Owner's Manual page 521

It is recommended that the vehicle is taken to your
dealer for service if the headlamps need to be adjusted.
It is possible however, to re-aim the headlamps as
described.
The vehicle should:
Be placed so the headlamps are 25 ft. (7.6 m) from
a light colored wall.
Have all four tires on a level surface which is level
all the way to the wall.
Be placed so it is perpendicular to the wall.
Not have any snow, ice, or mud on it.
Be fully assembled and all other work stopped
while headlamp aiming is being performed.
Normally loaded with a full tank of fuel and one
person or 160 lbs (75 kg) sitting on the driver seat.
Have the tires properly inflated.
Have the spare tire is in its proper location in the
vehicle.
Headlamp aiming is done with the vehicle's low-beam
headlamps. The high-beam headlamps will be correctly
aimed if the low-beam headlamps are aimed properly.
To adjust the vertical aim:
1. Open the hood. See Hood Release on page 6 15
2. Locate the aim dot on the lens of the low beam
headlamp.
3. Record the distance from the ground to the aim dot
on the low beam headlamp.
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