To change the definition of any metric, including the time period used for the calculation, go to 'Settings', 'Metric Library' and click on the edit button by the metric you wish to modify.  (edit button is circled in red on the below screenshot)

This will open up a screen enabling you to change all aspects of the metric definition: the calculation rules, the input definitions, time periods, etc.    See screenshot below.

Remember to Save your changes once made.