If you're goint to show psudos instead of real name in K2 items, there's already a subjet in the forum, however :
If you want to ALSO have pseudo to show in Module K2 Content, then go to :
However, I need to add that the above modification is made in the core module file and not the override which is not recommended unless absolutely necessary.
This means that you need to backup your file before you make a K2 update, otherwise, your changes will be lost.