To update the currency of a self managed supplier please follow the below steps:
Click into the Admin Cogs along the top bar.
Click into Suppliers then into Manage Suppliers.
Click into the Self-Managed tab at the top of the page and locate the supplier and click the Magnifying Glass.
Scroll down on the details tab to Geographical, Tax and Currency Settings and update the currency in the Invoice Currency drop down box.
Click the Save Changes button at the bottom to confirm the change.