First things first, make sure you're on the Credentials page.
Scroll down to the Assessments section.
Find the Assessment you'd like to delete, then click the Edit (pen) button or Upload button, depending on the status of your assessment, to open the edit form.
At the bottom of the form, click on Delete Assessment.
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.