I have two Kohler Low Boy Toilets. One works fine, the other seems to continually run very slightly. When I hold the ball down to manually fill the toilet, it will shut off---otherwise it seems to run very slightly for five minutes or more. How do I adjust the water flow valve?

Sounds like the 'Ballcock' the device that allows water to fill the tank and then shuts off is the culprit. It needs to be repaired or replaced. When it makes noise or fills for minutes at a time the flow has been restricted by debris and worn seals. Shut off the water to the toilet. Drain the tank. Remove the water supply line and then the ballcock. Some mistakenly refer to it as a fill valve, which actually is the part that allows water to move from the tank to the bowl and cuts off with a flapper. Take the ballcock with you to your local hardware store. Kohler 'lowboy' type toilets are not as tall as the most common replacements. Take yours with you to match it with a good replacement. Its not a very difficult job. I'd recommend replacing the water supply line at the same time. There are a variety of different Kohler ballcocks, and rebuilding them is an option; but availability of parts can be a problem.