Have a look at this part of the help center: I accidentally created two accounts; how do I merge them? It explains what you should to do merge two accounts.
See also:
- How can one link / merge / combine / associate two accounts / users? (Anonymous / unregistered / cookie, or Google / Facebook / registered) (Meta Stack Exchange)
- How do I combine my two accounts? (Meta MathOverflow)
- Announcement: New User Merge Policy/Tool (Mathematics Meta)
- How can I unify/merge two of my accounts? (Mathematics Meta)