If for what ever reason you need to get into the Windows Start-up Settings, including safe mode in Windows 8 – (this used to be advance boot options) then try these options:
#1 – “If you're not signed in to Windows, tap or click the power icon from the sign-in screen, hold Shift, then tap or click Restart”
#2 – Head into your PC settings to find Advanced start-up (Detailed instructions on how to)
Once into Advanced start-up you can do things such as start in:
Read more about each option at Windows Start-up Settings (including safe mode)
do note however that If you're using BitLocker on a tablet or Windows RT device, you'll need to suspend BitLocker before you can get to Windows Start-up Settings.