1. Connect the Kindle Paperwhite to a computer USB-cable.
2. Create an empty file in the root directory DO_FACTORY_RESTORE (This need to be just a file without any extension). Follow steps below to show and remove an extension, if it is hidden
3. In Windows Explorer, choose Tools > Folder Options.
4. Click the View tab in the Folder Options dialog box.
5. In Advanced Settings, select Show Hidden Files and Folders.
6. Deselect Hide Extensions for Known File Types.
7. Click OK.
8. Safely disconnect the reader from your computer. Do this by selecting USB in your taskbar and clicking Kindle under "Safely remove the device". You can keep it connected physically if you want. 
9. Reboot your device by holding the power button (about 40 seconds).

It will take you thru setup steps and re-register as you would have done the first time.