Hey scarrott,
Thanks for your post and welcome to the QuickBooks Online Community!
Currently there is no way to edit the customers details directly when entering the invoice, if you need to update the details without moving away from the invoice you can right click and duplicate the internet tab you are working on, press Ctrl + Alt + C to bring up the customer list, then filter the customer in the search bar and edit.
If you would like to leave a feature request for this please leave feedback within your account by going to the Gear icon > Feedback :)
