Here is Asus's instruction:
While the computer isn’t powered on yet, press and hold the [F2] button of the keyboard, and then press the [Power button] (Do not release F2 button until the BIOS configuration display.).
Note: For certain older models of desktop computers, it may be necessary to press and hold the [Del] key, and then press the power button to start up, in order to access the BIOS configuration.
Quoted from Asus' official website: https://www.asus.com/support/FAQ/1008829/#:~:te....).