In the invoice list edit an invoice and click on the client's name

A new window with client's details appears

Change client's data and click on Save

Select No to change the data only on the invoice
or select Yes to change also client's details saved in the Client section

Change client details in the Client list

Edit the document and remove the client by clicking on X

Add a client and select the corrected client from the list

Save the document

In the Client's list edit the client and change its details - name, address, e-mail

Edit the invoice and select another client (any client will do)

Save the invoice

Edit the invoice again

Select the right client

Save again