Thanks for reaching out to the Community!
I am not too sure what you mean by “as the contact has not subscribed I cannot see how to do this”. Where are you seeing this email address? In this case what I would suggest doing is adding this person to your account as a new contact. If you have already done so and simply need to update their email address then you can follow the steps Here to update that email address.
I hope that this does the trick! Have a good one!