There is no “downgrade” in Helm, but you can “upgrade” to an older version. This is not a safe thing to do though because there are etcd database changes between versions. The only correct way to revert is to have a backup from before upgrading, and then restore that backup.
In Helm, there is no way to “downgrade,” but you may “upgrade” to a previous version. However, because the etcd database changes between versions, this is not a safe thing to do.