Unfortunately, you will have to end the workout by clicking on the stop button and going through the process of creating a new workout again. While setting up the new workout, you’ll be able to select the correct pool length.
NOTE: If this happens before the Triton units are used in the pool there shouldn't be any issues. If you do this after the units have already been used that workout, there's a chance that the units may not immediately update to the correct pool length. The Triton Connect will make sure to correct the unit over time, but you may receive a few incorrect metrics in the interim.