Firstly, make sure you are signed-in as the user whose address you wish to update. Click on the profile picture in the top right-hand corner of the page and select 'Account Settings'.
Here, you will find the option to change the email address that is associated with the account.
Enter the new email address you would like to use along with the current password linked to the account in question.
If the email address is valid, you will receive a 'success' banner, as illustrated below.
To check that the email address linked to the account has been updated, head to the Users page and search for the user in question, where the new address should appear next to the user's name.