Short answer
HubSpot's KB: once two records are merged, it isn't possible to unmerge them. The Merged record IDs property shows which records were merged in, and an export covers pre-merge property values and timestamps for merges up to 90 days ago. For contacts and companies, create a new record with the merged email or domain.
1. Merge menu
Merge lives in the record Actions menu in my demo portal.
2. Prevent it
Check both records before merging. See merging records.