You need to have space to download the files, oftentimes those files are compressed and need to be decompressed (expanded) so they can be put in the right spot. It would be a terrible day if the update just tried to replace files needing update as it downloaded them, because if *anything* happened – internet disconnected, your device restarts, etc. – then it could royally screw up how the device works. So it *has* to download everything and make sure it’s all there before it starts replacing/updating files.
Latest Answers