In order for you to update a Contact's data you just have to follow the steps below.
Hover your mouse over the Manage button in the header menu.
A drop down list will appear.
By selecting the option "Contacts", a new page will open up with a list of all your Company's Contacts.
Click on the Contact whose data you want to change.
Change the section/sections you wish to update and click "Update this contact", in order for the changes to be saved.
The Contact's data is now updated.