Follow these steps in the same browser window (Chrome is preferred for all Formitize usage on a desktop) and don't open any new browser tabs until you've completed them:
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. In the Formitize Web Portal, go to the Settings cog > Integrate > Integration Profile
3. Delete the existing Xero authentications listed here by ticking the checkbox at the bottom of the list 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.