There's no reason to enter the BIOS prior to restoring the factory image, assuming that's what you meant by asking how you can reinstall Win8.1. Like xXLeonXx posted, after pressing F9 and when at the Options menu, select Troubleshoot--> Reset your PC--> Only the drive where Windows is installed--> Just remove my files.
How to refresh or reset system on Windows 8 pre-installed Notebook PC ?http://www.asus.com/support/FAQ/1003728
Laptop: Asus G750JZ
OS: Win10