It is easy to change what a metric is called. Go to 'Settings', 'Metric Library' and click on the edit button by the metric you wish to modify.  (edit button is the pencil, circled in red in the below screenshot) Change the name of the metric by typing a new name into the box at the top left showing the existing name. Remember to Save your changes once made. 

Also see How to review and change the way a metric is calculated; how to edit a formula.