Hi @VinceMeurlott 

To get started from your Square Dashboard, navigate to Customers on the left side menu section, and then choose Customers again.  Choose the customer that you need to edit, their info will pop up on the right hand side.  Click the Edit on the right under Personal Information. Update their email address and click save.

I believe that there is a glitch in the system that doesn't allow you to edit/add a customers email once the invoice has been sent.  You can add it, but when you go to re send the invoice the system acts as if you never changed the info.  The steps above is what I use when i need to edit a customers email for an invoice that has already been sent. 

Let me know if this works for you.