How do you adjust the width of the beam of light?

Owner's manual says high beams will be adjusted by correctly adjusting the low beam.
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GM Owner Manuals (dealereprocess.org)

Headlamp Aiming :: Vehicle Care :: Buick LaCrosse Owners Manual :: Buick LaCrosse - BuiClub.com

The headlamp driver module (HDM) is an electronic module that provides electrical power to the vehicle low beam headlamps. The HDM is controlled by a pulse width modulated (PWM) ground signal from the body control module (BCM). The BCM determines the desired lamp intensity using the ambient light sensor, multifunction switch and the park brake switch as inputs. In general, the HDM will operate in 1 of 3 modes:
Your best bet have it checked by a qualified repair shop ! May have DTC'S diagnostic trouble codes .
DTC B2575
DTC B2575 Headlamp Control Circuit
When the body control module (BCM) senses a request for headlamp or daytime running lamp (DRL) illumination, the BCM sends a pulse width modulated (PWM) ground signal to the low beam headlamp driver module through the low beam headlamp driver module control circuit. The low beam headlamp driver module will illuminate the headlamps, adjusting the intensity based on the PWM signal received.
DTC B2580 Headlamp High Beam Control Circuit
DTC B2645 Ambient Light Sensor Circuit
The body control module (BCM) monitors the ambient light sensor signal circuit in order to determine if the daytime running lights (DRL) or the auto headlights (ALC) should be turned ON in the AUTO mode. When the BCM senses the ambient light sensor signal voltage is between 1.75-4.9 volts, either the DRL will be commanded ON or the ALC will be commanded ON depending upon the amount of light received by the sensor.

The headlamp driver module (HDM) is an electronic module that provides electrical power to the vehicle low beam headlamps. The HDM is controlled by a pulse width modulated (PWM) ground signal from the body control module (BCM). The BCM determines the desired lamp intensity using the ambient light sensor, multifunction switch and the park brake switch as inputs. In general, the HDM will operate in 1 of 3 modes:
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Reduced Intensity Mode: This mode is used by features that do not require or allow maximum low beam headlamp intensity such as daytime running lights (DRL) (nominal: 85% duty cycle).

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Full Intensity Mode: This mode is used by features that require full low beam headlamp intensity such as low beam headlamp operation in manual or auto modes (nominal: 100% duty cycle).

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Off Mode: This is the default mode of the system that will occur when none of the features described above are active. The BCM will deactivate its PWM output (0% duty cycle) during this mode.

The normal battery voltage input to the HDM must be between 6 and 16 volts DC to operate. Any deviation to these values could cause improper operation.
Major components of this system are the BCM, HDM, ambient light sensor, multifunction switch, park brake switch and low beam headlamps.
DTC B2575 Headlamp Control Circuit
When the body control module (BCM) senses a request for headlamp or daytime running lamp (DRL) illumination, the BCM sends a pulse width modulated (PWM) ground signal to the low beam headlamp driver module through the low beam headlamp driver module control circuit. The low beam headlamp driver module will illuminate the headlamps, adjusting the intensity based on the PWM signal received
You best bet is to take your vehicle to a ASE certified repair shop or the dealer >

The scan beam is fixed and can not be adjusted. However moving the n410 closer to the bar code will reduce the width of the beam itself. The narrow beam is designed to accommodate static stands as well as manual use.
Simply hold the n410 a few inches away from the bar code and the beam will be able to read the code underneath. No adjustment except the distance between the n410 and the barcode is required.

the bulbs are set in the reflector to get maximum illumination and cannot be adjusted. What you have to adjust is high beam and the low beam will be automatically adjusted. Th adjust high beam (after dark) place the vehicle on level ground about 3 feet from a wall. Turn on high beam and you will notice where the centre of the beam is on the wall That centre has to be slightly below the centre of the lights ( about 1 inch) The centre should be directly in front of the lights and not out to the side of the vehicle. When the lights are correctly adjusted then low beam will be in the correct position.

Take your car and park it 10 ft from a wall or a building, now take a tape measure and measure 36 inch's from the ground up and draw a line so its the same width as your car front end , turn on your head light and see if center of your lights our hitting the line you just drew , if not adjust them until they do. The right side should be just to the right a bite and the left one straight a head .

There will be no adjustment for low beam as the headlight assy. adjusts for both beams. Adjust lights on high beam from a distance of about 3 ft(1meter) The center of the beam to be slightly below center height of the light and slightly to the left (AUS) or right (USA). Face the car to a wall at night is best.

Buy a new TV, its not worth repairing an old CRT set

In a very level supermarket carpark at night, shine the headlights square onto a wall from less than 3 meters away. Chalk the height of each low beam's horizontal cut-off line on the wall. Then back the car up like 30 meters and the cut-off lines should now be lower but only slightly. Fiddle with the vertical adjusters until they look right. Temporarily cover the headlight you're not adjusting so it's really obvious which one's line is which.

Then you can check the left-right aiming using the high beams. Don't try to use the vee of the low beams for this, every bulb will have its vee in a slightly different place. To pickup the car's centre line, crouch down behind the trunk and sight through the windscreens using an imaginary vertical line (drawn between the trunk lock and the rear view mirror's stem) as your reference, and chalk that vertical line on the wall. Your high beams should fall either side of that line, and be about the same distance apart as the width of the car.

Unscrew the bottom portion, where the batteries go. Then unsrew the top where you can adjust the beam width until it comes off. Very carefully pull the bulb out and replace the old bulb with the new. The Reassemble is the reverse of the procedure.