You can change your email address only if you remember your old password. Incase you have access to your old email you can reset your password and then change your email by adding alternate email.
If you do not have access to old email or password. I would suggest you to make a new ResearchGate ID and then merge it with the old ID to add the publications.
In short,
* Remember your password? Reset it and then change your email address within your account settings.
* Can't access old email address? Create a new account and merge them if possible.