To change the Rep or Lap metric for all athletes in a group:
From the dropdown at the top of the tile view, select the metric you want for the Rep or Lap
To change the Rep or Lap metric for an individual athlete:
In the Rep or Lap section of that athletes' tile, press the metric that's there and choose a new metric from the dropdown that shows up.
To change when the Rep metric is updated:
Go to Settings, expand the Athlete Tiles section and select either "Last Completed Rep" or "Current Rep, as it Happens". Last Completed Rep is the default which only updates the metric once a rep is complete. Current Rep, as it Happens allows the rep metric to update after each completed length.