At the same link, under Download/ Manuals, you can download the pdf version of the manual, go to page 2-14 where finding Clear RTC jumper (bottom of the motherboard, 3-pin jumper, labeled CLRTC).
Follow the instructions in the link regarding turning off, unplugging the power cord, moving the jumper. At startup, no need to press Delete. The system would ask you if keeping the default settings.