Hi Fong,
If the email is already in the queue and waiting to be sent, the email address would have to be edited there.
If you want to re-send a customer invoice by email and just want to use the new address, you can choose to email as usual and when you are asked which address to use, pick the 'Customer' choice instead of the 'Bill to' or 'Ship to' options. The 'Customer' option looks back at the customer record rather than the invoice for sending.
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Regards,
Softrak Tech Support