First things first, make sure you're on the Credentials page.
Scroll down to the Health Documents section.
Find the Health Document you'd like to edit, then click the Edit (pen) button or Upload button, depending on the status of your health document, to open the edit form.
A form will slide out from the right.
When you are done editing, 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.