If the customer has claimed their account, i.e., they've accepted the email invitation to manage their account and have created a password, you won't be able to change their email on their behalf. However, they can edit their email by logging into their account - here is a guide with steps you can forward them if they need help with this.

On the other hand, if the customer's account is unclaimed (you'll be able to see at glance if an account is unclaimed because there will be an 'Unclaimed' pill next to their name, as seen below)

you'll be able to edit their email address by clicking into their profile >> clicking the 'Account Management Settings' button and then editing their email address.