Something went wrong in the software or during an update, or you want to start fresh? Here is the reinstall option

Backup first your home map (/home/username) to your usb stick to backup all your documents and configurations)

Use a normal networkcable, not wireless.

Start the PC and immediately after the bios screen (or  from the actual start-up) press the two arrows up keys and the Esc key simultaneous  and let go with an interval of half a second.

Until you see this menu

Select the "herinstall" with the little arrow keys

If you have this entry, press enter

After installation do the first-run

Put your backupped files back in your :home/username directory