Q. When and if I put in the wrong meter reading for a monthly space rental, I can't seem to change it back to the original readings to correct it. Is this an unreversible action or can I change that?
You can correct it by going into the Site Details for the site -- right-click on the site, select Site Details, and enter the original ("last") reading. Also delete the incorrect charges from the reservation of course. Then you can enter the new reading/charges again.
If you don't know what the original reading was, look at the charges on the reservation before deleting them -- it will show the original reading (and the new one) in the description.