Once you have integrated with Xero and your invoices have been uploaded to Xero, you can access the invoice in Xero from Heydoc. Likewise, you can also access the invoice in Heydoc when in Xero.
To do this, go into the invoice and click on the ellipsis in the top right hand corner. Select "Go to Xero" from the drop down list:
This will take you to the record of this invoice within your Xero account. If click "Go to Heydoc", this will take you back to the invoice within Heydoc: