I need to make some modification to the title of item before it is saved and even before it gets into converting to an alias. I want to make change to it before it is used in any kind of processing by Joomla or K2.

Hope someone can suggest me a way or tell me where can I change it in php files.

Thank you...

It is better to do it as a k2 plugin by overriding the onK2BeforeSave function.

But if you don't care to hack k2 files, you can do it by editing administrator/components/com_k2/models/item.php at line 58, directly after this line:
You will check whether the item is new, then modify the title, like this:

Thank you for suggestion.

However, I need the value of the current title, so how can I get the current title?

P/s: I want to use str_replace to modify the current title.

Thank you.

$row->title is the current title

Thank you, it is solved perfectly now. :)