If you want to update any existing details on an already created Invoice, for example, you want to add your Customer GST Number then from here you can do that.
You can explore more options that you can update using this Popup, but make sure this update will just reflect on the invoice for which you clicked the update button.
Bulk Order Update Button
If you make any changes in your Shopify store, you’ll see a "Bulk Order Update" button—simply click on it to view the list of updated orders, then click "Update All Invoices" to apply the changes.