So according to the manual the unit is already set to achieve temps between 35-38 deg F. So I would counter with the question, what are you trying to achieve? Is it not making temp? I would clean off the condenser coil. Make sure there is no ice build up on the evap if either of those conditions exist, go ahead and shut the unit down. Defrost and clean the coil and see it it will make temp after that. Simply lowering the temp will not solve your issues. You may have a refrigeration problem. Let us know if this helps.
I noticed a post on the same Master-Bilt model last month, and I was wondering if you are the same user. If so, please consider hitting the link to register now. Registration is free and allows you to join the dialogue. Oftentimes we have follow up questions that can help you ensure your problem is fully solved and not just a temporary fix.
On to your issue, the manual does not specify that it is possible to adjust the temperature of a Master-Bilt MBGR24H specifically, but if it were possible, you would have to access the control parameters with an access code.
Unfortunately, I do not have this access code, nor do I have the technical information on the steps required to change settings once the code is entered.
I am looking at a “Bud Light Chill Box” model number MBGR24H-58 branded as a Master-Bilt, but actually built by Ojeda Refrigeration. This is a half-size glass door reach-in programmed to run 22-24 deg F. I am surprised this question appears to reference a nearly identical model number, but is programmed as a conventional RIC?
The AH is a high temp alarm. So something is not quite right with your cooler. Start by inspecting the coils for obstruction and air flow. Evaporator may have froze up from high humidity or fan speed. Door gaskets are a big cause of this. The condenser fan speed and the air flow through the condenser has to be sufficient to keep the compressor head low enough for efficient operation. This also includes clearances for the case venting. And with some vertical reach-ins, air flow baffles to direct air in on one side of the bottom front and out the other. Especially check the coils with a light to be clear in the center. I've seen many that the outside was clear visually, but blocked deep inside the coil requiring a foaming clean. Which also requires a copious amount of water.
How old is your cooler? Some beer distributors retain ownership of those coolers and will repair or replace them. Providing they service you.