Likewise I am unable to find any definitive answer as to how to adjust the low disk space warning and I suspect that that is a good thing since the warning is provided for your information and protection to advise you of an impending problem with available space on your disk / partition.
Most answers involve addressing the problem directly either by replaciing the disk with a larger one or by performing some type of housekeeping either by disk cleanup or moving some of your data to a new location (another partition or drive) if possible.
If you can provide a screen shot of your disk management console (snipping tool, attach to your next post) perhaps someone might have some suggestions as to how to go about addressing your particular problem.
Keep us posted.
Randy