Do you have a problem? Is it working right? If you don't have a problem, leave it alone.
If there is a problem, call Rinnai and Have them give you some names of Authorised Service Providers. Choose one and have them come out. If you try to reset the manifold gas pressures without the proper equipment and training, you will mess it up worse and posible void the warranty.
What makes you think it's running rich? I get the gas pressure thing, but have seen many like that never need anything done.