Assuming you wanted to flash BIOS under 64-bit Windows OS,
1) Extract the FPT_Z390_1123.zip, pick the 'Windows64' folder. Extract the folder somewhere, for example C:
2) enter an Command Prompt (cmd) as administrator , navigate to the previously extracted 'windows64' folder
3) enter command FPTW64 -f BIOSNAME
(BIOSNAME is your bios files name with extension)
wait until done, after that shutdown-clear CMOS, and its's ready.