In order to make changes to an invoice you need to be the owner of said invoice and the invoice must be in an editable status (N, IP, RE, R). You can take ownership by clicking in the 'Owner field' to activate the user list. Start typing in your name and when you find your name select it by clicking on it to change the owner to yourself.
If you cannot change the owner it likely means you have insufficient permissions. Please contact your system administrator for assistance.