How do I update the group date format that appears when items are grouped by year and month?

1. Access the preferences pane.
   
   Keyboard shortcut: Command key (it has a ⌘ symbol) and then the ","
   
   
   
   
2. Select from the list of formats in Group By popup list in the Date Format section.
   
   
   
   

If you don't see the format you would like, please let us know and we will try to add it in a future update.