First things first, make sure you're on the Credentials page.
Scroll down to the References section.
Find the Reference you'd like to edit, then click the Edit (pen) button or Upload button, depending on the status of your reference, to open the edit form.
When you're done updating, don't forget to Save!
To cancel any changes you've made, simply click the left arrow in the top bar. You will be prompted "You have unsaved changes. Are you sure you want to leave without saving?" Just click 'OK' and your changes will not save.