when octobercms updated, all changes of /backend are deleted? for example: if i alter login page of /backend, in your update this change will stay?

Depends on how you modify them. If you're changing core files under /modules or /vendor, updating is going to wipe out those changes which is why you should never ever touch any of those files. There are a lot of different ways to customize pretty much everything in October to such a deep extent that you shouldn't ever need to touch any of those files.