You can change customer for existing invoices on device if the appropriate option is turned on for their type in the Back Office application.
To turn on the ability to change invoice customer on device
In the Back Office application, on the Company tab, click Invoice Types.
On the Invoice Types page, click Edit next to the appropriate invoice type.
In the Invoice Type dialog box, select the Allow Client Changing check box.
Important! Device users must update the main database on their device to get the changes applied.
To change customer for an existing invoice on device
On the Home screen, tap My Invoices or Team Invoices.
On the My Invoices screen or the Team Invoices screen, tap the appropriate invoice, and then tap Change Customer.
On the Customers screen, tap another customer.