The only way to gracefully (keep configuration) downgrade an SZ node or cluster is to restore a cluster backup taken prior to upgrade. This is why saving a cluster backup prior to upgrading is highly recommended and suggested when you initiate an upgrade.
You need to verify the upgrade path for each new version to determine if you can directly upgrade to the new version: https://support.ruckuswireless.com/documents/3139-smartzone-5-2-0-0-699-ga-upgrade-guide
Upgrade from 3.6, 3.6.1 or 3.6.2 to 5.2 is supported, but all saved statistics will be purged (not configuration)
The only other way to downgrade a cluster would be to factory default each node. There are two steps in setting up a fresh install. The first setup configures the IP addresses and network and can be done from console, CLI or WEB-UI The second step creates or joins the cluster. If you do the second setup from the WEB-UI you will see a button on the bottom of the create/join page that permits you to upgrade or downgrade to any version without checking existing version. This allows adding a new node to an existing cluster by matching the version of the existing cluster.
Of course - if you go this route you will have to re-create your configuration from scratch unless you saved a configuration backup for the version you wish to run.
Since you have a cluster you will essentially breaking the cluster and upgrade/downgrade each node. The first node you will create a cluster and the second node you will be joining the existing cluster. You can configure the first node and then join the second node which copies the configuration or create the cluster and configure the system as a cluster.
Sorry for the inconvenience this has caused