Try mashing F10 and Delete when you boot it up.
Alternatively, you can locate the CMOS Jumper on your motherboard. This will make the system go directly to the BIOS after you turn it on.
If the BIOS isn't up to date, go to the ASUS website on another PC and download the latest BIOS or use provided updating software.
Put that BIOS updating file onto a pen drive and either..
1. Go into BIOS and change your settings so your PC boots from your pen drive.
2. If you can actually use windows, opening the file inside the pen drive while in windows should allow it to update.