How to update a existing customer?

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Hello,

When we create a supplier, a customer record also is being created automatically. We want to upload customers to the system. 
But since we have to create the supplier first in case that customer will be also a supplier. How can we upload a customer info, such as account, site, reference accounts after its created?

Also, can we add bill to site and ship to site purposes at the same time?

Thank you.