To change the email of a User, hover your mouse over the Manage button in the header menu.
A drop down list will appear.
By selecting the option "Users", a new page will open up with a list of all the Users of your Company.
Click on the Contact whose email you wish to change.
A new screen will appear. On the bottom of this new page there is a field where appears the email you want to replace.
Complete the field with the email you want to add.
Once completed, click the "Update user" button to complete the request.