If you have connected the Xero integration, invoices created in Collums are synced to Xero after 5 minutes. Once this has happened, you will see a blue Xero icon next to the invoice name in the invoice pop-up.
Your invoice will re-sync if it is changed in any way but if you wish to re-sync it manually, in the invoice pop-up:
Click Options > Re-sync to Xero