I ran into this issue too today.
Hopefully you made the backups that they tell you to.
Go and check your version. Check to see if your running 9.0.1 or 9.0.2. From there, make sure you install the right hot fix.
To roll back, you will need to restore your cfide folder that it told you to back.
You will need to restore the /lib folder that it told you to backup.
I had 9.0.1 and I was installing the 9.0.2. I didnt realize that 9.0.2 wasn't a "hot fix update" and it was the cf without solr version.