To change a metric that you have added to your Item, navigate to the Item's Metrics tab.
Click on the edit icon in-line on the metric you wish to edit:
This will open an edit window to allow you to change the metric's title, value*, date stamp for that value, and metric format. Additionally, you can edit the metric's target goal, value, and date:
Once you have made the changes you desire, click 'Save Metric'.
*Note that if you make an edit for the Value, but do not change the Metric Date, it will override the previous Metric Value