I am using k2. It seems to me meta description for each k2 item is automatically generated by getting a small part of the item. I think meta description is essential to SEO, isn't it? Is there anyway to manually edit it?
As well as finding it in Components >> K2 >> Items >> Click in article item >> Tab "Meta data information". as mentioned in the other answer, you should also understand some other places where the meta data could appear.
Joomla! is a menu based CMS, and as such the menu item takes precedent when choosing Meta Tile and Meta Description. If you have a menu item that links to the k2 article it will override the K2 Meta Title and Meta Description.
You can find these items under Menus > Click on the Menu Item:
You can also effect your meta settings in the Global Configuration.
All of these items should be carefully filled in, taking care to avoid keyword stuffing, being aware of where they appear on Google (the page title is the blue link, the meta description is the text below the link). Consider how they will affect users, whether they will encourage click throughs, and whether they contain your keywords.