Find the client you want to delete a measurement on the Client Index page. Hover over their row and you'll see a small graph icon appear on the far right side of their client row.
Click on this icon and you'll be taken to the client's metrics page.
Find the card for the metric and click on the list icon. Scroll down to find the data point you'd like to delete and click the trash icon.
Confirm your selection and the data point will be deleted.