customers from the drop down ist will not transfer to invoice

[American Dispatch]

My invoice has tabs for the invoice and customers, my invoice has the drop
down list that i select and it places the company on the invoice to section.
My new girl in the office hand typed the company in the invoice to section,
and now , when i select a company from the drop down list, it will not
transfer the company's billing info to the invoice, how do I correct this ?

Thanks