Board Thread:Support Requests - Getting Started/@comment-38012622-20190101203114/@comment-1038387-20190101203703

Try logging in with the old account, and select the Forgot Password option. It should send a login link with a psssword reset to the email address on file. If you don't get an email, send a report to Special:Contact/account-issue

You cannot actually merge two accounts. You can choose to close one, via Special:CloseMyAccount. Be careful, this is permanent.