Go to Downloads > Products > [K2 Five | K2 blackpearl]
If you are on K2 blackpearl 4.7, go to the page for the Cumulative Update installer. If you are not already on the latest Cumulative Update, download it and install prior to fix packs.
If you are on K2 Five, go to the Full Installer page for your version (Five RTM, 5.1, etc.).
Select the relevant fix pack from the list on the left side of the screen.