I believe if you do a hard (factory) reset it will clear all saved data and restore the unit to its out of the box condition. That might be a good idea anyway with a used unit since then you can ensure everything is set up the way you want it to be. If you do not have the latest software on the unit, update it before doing the reset.
You can find the instructions for doing a factory reset on the Furuno USA site.
You can also go into HOME > SETTINGS > POINTS & BOUNDARIES
then scroll to the bottom and DELETE ALL OBJECTS. It will delete everything, points, tracks, routes, any object you have saved.