Various ways.... Some would simply delete one of the units and make the 1 unit the size in the system. That's if it's a more permanent change and not expected to be shrunk back down to 2 10x15's in the future. Another would make one unrentable and set the price of the rentable unit to the proper price and put a unit note "merged with unit #123". This is more common when it's a virtual conversion and 2 units are rented even if they're not physically connected.
Answers