Navigate to your campaign's settings area via the left sidebar, click Custom Metrics from the settings page menu, then click the name of the custom metric you would like to change.

Alternatively, you can edit the custom metric from your dashboard or report directly by clicking the ... in the top right, then selecting 'Edit Custom Metric' from the sidebar.

This will pull up the Update Custom Metric page, where you can edit the metric formula, and other fields. To save, click Update in the bottom left of this page.

Once saved you'll see the custom metric widgets update to reflect the changes.