Get a customer
$customer = Get-PCCustomer -tenantid '<tenant id GUID>'
Get customer Billing Profile
$customerBillingProfile = Get-PCCustomerBillingProfile -tenantid $customer.id
Set customer Billing Profile
$customerBillingProfile.firstName = '<first name>'
$customerBillingProfile.lastName = '<last name>'
$customerBillingProfile.email = '<email>'
Set-PCCustomerBillingProfile -tenantid $customer.id -billingprofile $customerBillingProfile