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.
Simply enter the address you would like to use along with the password linked to the account in question and, if the address is valid, you will receive a 'success' banner as illustrated below.
The confirmation email should be sent to the address instantaneously; if this is not the case or the email is sent to a spam folder, please let us know at firstname.lastname@example.org.
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.