If the invoice is being sent to an exisiting contact you are able to go into their contact card and edit or add in the new details. To do this in your Web Portal > Web Portal > CRM > Contacts > Select Contact > Edit > Enter Details > Save. 

If the exisiting contact has multiple addresses or people you will be able to select this when creating an invoice. You are also able to add in a new address for that contact in the location field in the invoice. 

If it is a new client you will need to create a new contact before creating an invoice. To do this in your Web Portal > CRM > Contacts > + New Contact.