Just tap on a watch to edit it; you can edit the information about that person or race, add personalized notes for that individual for that race, change the type of this individual watch, and edit any of its times.
If you edit a lap time while displaying split times, then it will affect the total length of the race. For example, if you add three seconds to a lap, then the other laps are not changed, so the total race time will go up by three seconds. On the other hand, if you edit a lap time while displaying cumulative times, then that will only move the boundary between that time and the next lap.
If you change the time of a running watch, say to subtract 3 seconds, then the watch keeps running during that period, with a net change of 3 seconds no matter how long you take to edit the watch. So if you hit a lap button late, no problem, just take a few seconds off.