Open your address book from within Outlook by clicking on the address book icon as shown below:
The Address Book dialog will open
You can navigate your address lists with the drop down menu
You cannot add/alter someone in the Global Address List as it is managed by the Exchange server - but you can modify the data for a person in the Global Address List after you copy them to your Contacts list. To do this:
Right click on the persons name in the Global Address List OR double click their name and click the Add to Contacts button
Select Add to Contacts
The Contact dialog will open allowing you to add/alter any fields you want
When you Save this entry, this person will be copied to your Contacts address list with the altered information
You can now reference the altered information by looking at your Contacts address list