To edit default fields you need to follow next steps:
Click on Account submenu from CRM module.
Click on
icon from the right side of the page then click on CRM settings.
Click on default field's
icon from the right side of the page then click on Edit.
For adding new values click on
icon, add text into Value field then click on Save button.
For edit an already existing value click on value's
icon then click on Edit button.
Edit the value then click on Save button.
For deleting a value click on value's
icon then click on Delete.
Click on Yes.
For doing the same thing to contacts, before step 3 click on Contacts submenu then follow every steps from above.