Depending on your K2 version, you should grab the related zip file and extract it inside your /templates/myjoomlatemplate/ folder.
If you're on a Mac, be careful not to override the /css, /images and /html folder. Copy the zip file inside your Joomla! template's folder, navigate with the terminal there and then run: ~ unzip K2_v2.x_override_package.zip
These packages are available so professional (and not only) template developers can use to compare any changes in the K2 files so they can adapt their own custom K2 overrides. Use free programs like FileMerge (included in Mac OS X), WinMerge (Windows), KDiff3 (all platforms) or DiffMerge (all platforms) to pinpoint the changes from version to version.
This post will be updated constantly as new K2 versions come along.