Yes, you can update a member's email address!!
Take a look how!
From their profile in your members page on your business web portal, head to their details tab.
At the bottom of their personal info you will see a button "Show Options". Tap that and a drop down will appear.
Use the drop down and select the option "Update member's email address".
One you select 'Update member's email address' this pop up will appear.
Change their email address to their new one and you will also need to update their password as you are updating their log in credentials.
Once you have updated both hit 'Update email address' to save. Once you do let the client know. Your member can either update their password from the account details page of their member web portal or they can simply work through the 'forgot password' prompt on their log in page and an email will be sent to them to update their password themselves.
Once they update their password they will use their new email address to and new password to log in.
I hope this helps!