The "update files only" are for windows, and contain the files that have been changed or added since the last update. If you have the previous version, you can use the update files to update the game without having to download everything again.
It's possible to use multiple update files, as long as you use them in order, but if you missed a lot of content, the safer option is to re-download the full game and move your save files from the old folder to the new game folder.
Both things are explained here: