Steps to revert flow to a previous version :
1) Grab the flow / sub-flow sys_id
2) Navigate to sys_update_version.LIST
3) Search with flow / sub-flow sys_id
4) Click & open the record previous/desired version to which you want to revert.
5) Click on form context menu UI action "revert to this version (revert to a previous version".
6) Check with the flow, it will be reverted to the previous/desired version.
Regards,
Ishaan