First of all connect your GPS unit to computer. Go to my PC. Then locate your Gps (say drive F:) Look for Garmin File, then look for GPX file. All your caches will be listed. Delete the files and picture files you do not need. Good luck!
I have been asking that questions ever since I bought mine many months ago. But the funny thing is that I have never needed to actually delete any. Over time, I have downloaded many times the capacity of the eTrex20 and there has never been a problem. If you really want to know, you may have to contact Garmin. Please let me know if you find out! MessieurPhil (email@example.com)