You can only revert a machine after you have started that machine.

Reverting through Kali Linux

To revert a machine while connected through Kali, click on the MORE menu at the top of your screen, then choose the Revert option.

Reverting when connected to the VPN

While connected via VPN, the command to revert is located on the far right of the lab machine row.

To revert a machine, use the command immediately to the right of the start command.