To edit an existing chart simply go to the the Analysis tab in the main navigation bar and click charts. Click on the gear icon and select "Edit" to edit the existing chart you would like to edit.
Next, click into any section that you would like to edit in the top right-hand corner of the screen. You can change the audience, data points, measurement, or visualization. You can also edit the design settings under "Visualize" screen. Lastly, you can edit the chart name by clicking the pencil icon next to the chart name.
Once you've edited you chart, click the "Publish" button to save and publish your chart. This will also update the chart if it is on any dashboard in your account.