Follow these steps in the same browser window (Chrome is preferred for all Formitize usage on a desktop) and don't open new browser tabs:
1) Go to Xero (or follow this link to Xero's dashboard https://go.xero.com/dashboard/) and click "logout" in the top-right corner's dropdown to be sure you're logged out from Xero.
2) Go to the Formitize Web Portal > Settings cog > Integrate > Integration Profile > https://service.formitize.com.au/integrate/profile
3) Delete the existing authentications listed here by ticking the checkbox and selecting "Delete" from the bottom right actions dropdown then "Go".
4) In the top-right corner, click "+ New Profile" and select "Xero" from the first dropdown and then "create new profile". It will then take you through the Xero authentication steps.