I don't believe it's possible to make any changes to the pre-built worklist.  If you want to make any changes to it you'll need to make a custom one.  I had just recently asked about how to change column widths and that was my answer.

Hi,

It's not possible to change the column names yet but you can change the column widths if you are using K2 Blackpearl 4.6.11.

You will find more about changing the Worklist column width here: K2 Column Worklist

Shah,

Please log a ticket as a feature request to change header name.