I take it you mean Tracks, or distance?
The eTrex records a lot of ongoing data. If its a track, then go to the menu with Waypoints at the top, scroll down to Tracks, save or delete the track log as required.
If you've saved it, then delete it, so you start afresh.
If its distance, go to the menu with the compass on, using the ENTER button on the left, press, the scroll to the corresponding menu, where you can clear the time and distance.
I hope that helps.