The info that you seek (and more) is here
Just scroll down to the section starting with
Erase Internal Memory
1. Go to settings
2. Select modem
3. Select advanced
4. Put a check mark in Use custom isp box
5. in the ISP Password box type (wash me.) without the parentheses
6. Then select done and it should wipe the book.
Make sure you include the 'period' in "wash me.
" (a total of 8 characters); this should "de-register" your unit!