Windows Server Downgrade
This is not something that's built in to the OS installation.
Your two options are:
Use snapshotting at the hypervisor or storage layer if this server is a virtual machine. Roll back the snapshot on failure.
Take a full system backup that is capable of being used for a bare-metal restore. If the upgrade fails, restore from this backup.
There is no Windows built-in way to downgrade an installed OS version, except for uninstalling service packs. This is not your case, as you are asking for a significant major version rollback.
You may be able to downgrade functional levels of certain components, particularly after installing new servers with the older version - but definitely no way back for the entire OS short of re-install.
See MDMarra's answer as well for external ways to manage your image. Options include snapshotting VMs, dual booting, native-boot from VHDX, backup...