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I had signed up with an account. But i do not have access to that anymore. I need to change the login id of my stackexchange account.
You can try the following:
Recover your password by going to https://superuser.com/users/account-recovery and entering the e-mail address that you used for your old account. You can then re-activate it.
Write to the team under https://superuser.com/contact to have your two user profiles merged.