Suggestion For Rescue Flash Drive For Working System

I just had my System bricked by an update. What is the best way to create a rescued flash drive for
the system after installation? I was able to restore my system with an older ISO DVD, but flash drive
would be better for the system .

Also how do you really set the kernel to boot from? clr-bootmanager on the DVD did not seem to really work? I notices after the swupd repair, In noticed the boot directory had syslinux.cfg file, tried to change
default line in that, that did not seem to work. It seems like the real control for booting is somewhere else. systemctl boot image?? maybe?