Page 3-49 and 3-50 in the owners manual.
Hold the button to bring up the settings on the MFD (as Shemeel said) and then go to DATE --> DATE --> Use the up/down triggers to get to the time area, highlight the hour. Hit the middle trigger (I/Set) to select the hour, then pull the down trigger to change the hour down.
For better directions and screenshots, see page 3-49 and 3-50 in the owner manual. Keep in mind the OM is also available on your "My Subaru" page on subaru.com.
You might have to be parked to do that.I can't figure out how to change/set the minutes.