In the Game folder within the chapter, there will be a placeholder for each of the old video packs, literally named OLD_VIDEO_PACK_ACT1.rpa. But the size of that file will only be 5kb, as its not really anything. What you need to do is download the new one (probably to another location) and then copy and paste it into the game file, and overwrite the one that's already there.
After you do that, the file will have the same name, but it will be 3-5 GB in size instead. Once you see it that way, it really should work.
There isn't yet a pack for ACT3 available yet for the newest chapters, unfortunately.