The only way to change someones e-mail address is to ask that person to change it on their own.
Why can't I change someone's email address?
You started using Productive and invited other people from your organization to join. E-mail addresses you put down when you invite people becomes their credentials. Once they accept your invitation, they will setup an account password.
So the next time they login, they will be using that same e-mail address the invite was sent to and a password they had to choose as a first step of creating their account.
That's why you can't change others e-mail addresses. No matter if you're an admin, the e-mail address works as a username and no one can change it except the person who's account it is.
They need to go to their profile Settings...
... and make changes themselves.