Once the contract has been checked out, the customer data can no longer be edited. Changing this data could cause the signed contract to become invalid.  However, it often happens that the customer still wants to modify the billing data after check-in. 

The best way to do this is by creating a pro forma invoice in the name of the customer listed on the contract, and then editing the customer data on the pro forma invoice. 

To create a pro forma invoice from the contract, you can create the invoice as you normally do: by clicking the '+ create invoice' button. But instead of choosing 'confirm invoice' on the next screen, just click 'close'.

Then go to management to invoices, where you will find the pro forma invoice in the list of invoices. (If you do not see the invoice, make sure to check the settings of the filter bar on the left)

You open the invoice on the general tile where you can find the customer data. Here you can delete or edit the customer data and add the correct information.