You can't change the email address, but you can add a user with the new address and delete the old one.
In order to be able to change it, you need to have admin permissions in your account. If you don't have them - ask one of your account admins to assist you.
Go to the account settings (cogwheel in the top right corner) and there go to "User Management".
Scroll down to the bottom of the screen, and click on "Add User" to add a user with the new email address and grant it the relevant permissions (User/Admin).
Click "Save" to apply the changes.
Log out, and use an admin access (of the newly added user or of any of the other admins).
In the same "User Management" view, click on the "options" icon next to the user you wish to remove, and then "Delete"