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