I don't think it is possible to move, via the upgrade path, from 8.1 to 9.0

The most likely move will be to do a CLEAN install of the OS.
Boot from the OS 9.2 disk and at the first Option button, choose Clean Install.
Follow the instructions from there. At the end of the install you will reboot into OS 9.0
You next move should be to upgrade from there to OS 9.2.1 (at least) OS9.2.2 is the highest version of OS 9.
Upgrades from 9.0 to 9.2.2 are free and can be downloaded from Apple.com

Let us know what else you need.