Since the ROG Strix Z370-F Gaming doesn't support bios flashback, you'll have to install your 8th gen cpu to update the bios.
ASUS recommends bios 1803 or later for the i7-9700, might as well update to the latest bios which is 3004.
How to update the bios...
1) Insert a usb flash drive into a usb port on your pc.
2) Download bios 3004 - https://rog.asus.com/motherboards/rog-strix/rog-strix-z370-f-gaming-model/helpdesk_bios/
3) Double click BIOSRenamer and extract the bios files to the usb flashdrive.
4) Reboot and tap the DEL key to enter the bios.
5) Click the EZ-Flash tab.
6) Locate and click on the bios file, it will ask if you want to update, click yes.
7) Reboot and update the Management Engine Interface driver (found under Chipset driver).
😎 Shut down your pc and install your i7-9700 cpu.
Here is a video on how to put the bios file on a usb flash drive.