After I flashed FydeOS, how do I restore my USB drive?

You can use a disk management tool such as Disk Genius to delete all the partitions on the USB stick and create a new one, formatting it with the file system you want.

A simpler solution is to use balenaEtcher or rufus to flash a single-partition iso image to the USB stick to restore from the multi-partition state created by FydeOS image.