Skyrim Update 1.9 Without Steam: A Guide for PC Users
If you are using the latest version on Steam, download the AE build. The older build supporting 1.5.97 is still around to support people who have downgraded their installation or otherwise prevented updates.
Skyrim Update 1.9 Download Non Steaml
Version 1.4.21.0.4 was released on 5 February 2012 (released for consoles on 9 February 2012). This is the same version as the third beta build. Those that were participating in the beta when this build was released would not have to download the update when it was officially released.[16]
In case you're not aware yet, the CK for Skyrim got a long needed update today to eliminate the restriction on the size of files you can upload. Along with this, the upload form in the CK got a couple of minor tweaks to include a small space for a description as well as a box to input change notes for an update. The game's official launcher has also been updated (along with one steam_api.dll file) to go with this as the Workshop downloads will be sent through the Steam client now instead of the launcher. Both modules must be updated to make this work. It's currently in beta, so you'll have to select the beta options from Steam. Details here: -workshop-file-limit-is-now-limitless/
An alternative solution is to launch Steam in Offline Mode or play without an internet connection. This would make you unable to download updates for any of your games, but would ensure that your downgraded version isn't accidentally erased by a surprise update.
If you intend to install any SKSE-based mods which contain native code (.dll files), you will need to ensure that you download the version of those mods appropriate to either SE or AE. Some of these mods have not been updated for AE at all, and others are still in development. See this Reddit thread for further discussion of related issues.