stupidcreature
To update again, you will need to wait for a new release, you can't rollback. However, keep in mind the update may take a bit too boot.
Does the phone boot into fastboot? If so, what color is the fastboot triangle?
Does it show the GOS logo? Then you should wait a bit, took me a few minutes personally on my 6a to boot. This can happen with updates here and there. After the first boot, it will go back to normal boot times.
You could also try sideloading the update again.