I am sorry I missed this post. Unfortunately, you can't undo the upgrade and there is no "hard reset" function on the Enduras.
Are you located in the United States? I would like to refer you to the correct support person for your area, but your profile doesn't indicate where you live. Let me know and I will try to help you.
Lowrance Lady Lowrance Pro Staffer, just trying to get the facts on Endura straight
I live in Russia and my Endura have been bought by ebay like used device. I haven't any papers or receipts for it and, as far as I understand, it will be impossible to send my unit to factory.
Anyway, I can use Ozi, iGo and another navigation programs on this device but unfortunately I can't recalibrate the compass.
The only thing which I can now - wait new firmware. May be new one will fix this problem.
Thanks!