To downgrade a SPIKE LEGO® Education SPIKE™ Prime hub, you need to use the Hub Downgrade tool: https://spikespikelegacy.legoeducation.com/hubdowngrade
To use the hub downgrade tool, you will need to use a Chrome Browser on ChromeOS, macOS, or Windows.
Follow the instructions in the hub downgrade tool to downgrade your hub. Once it has been downgraded, it the hub will need to go through one last upgrade step inside the SPIKE Legacy App to be useable in SPIKE Legacy.
...