Hi Pegah,
You first need to set the Joomla Core Media Manager to the K2 media folder:
Joomla Core Media Manager > Options (located off to the right)
Enter "
media/k2" into the "
Path to files folder" and "
Path to images folder" parameters
Screenshot
imgur.com/mA2dh8x
Joomla Documentation
docs.joomla.org/Help34:Components_Media_Manager_Options
Note: If you want to start at the cache folder, use "
media/k2/items/cache" in the parameter instead of "media/k2"
I hope this helps,
Joe Campbell