Hi there mdeshaies-e3k-co,
Thanks for reaching out about your customer list. QuickBooks Online is a great tool able to import new customers through a file. I can provide some information about editing customers.
The import of customers can only be used to import new customers. It can't be used to edit customers, as when a customer with the same name is in the file, the line isn't able to be imported. To modify each of the customers, you'd need to go to the Sales tab, then Customers. Click on a customer that you'd like to modify to open their Profile, then Edit. Make the changes wanted, then Save.
If you have any other questions, feel free to reach out here.
