Navigate to the Goals tab of your Campaign Overview, by clicking Goals in the top bar of your campaign.

Alternatively, you can edit the custom goal from your dashboard or report directly by clicking '...' at the top right of the widget, then selecting 'Edit Goal' from the sidebar.

This will pull up the Update Goal page, where you can edit the custom goal fields. To save, click Update at the bottom left of this page.

Once saved you'll see the custom goal widgets updated with the changed information.