Follow this guide to downpatch to 1.6.353: https://www.reddit.com/r/skyrimmods/comments/18bh43k/if_you_accidentally_updated_your_skyrim_here_is/

To install, add a "Scriptable Auto Splitter" component to your Livesplit layout, and in the Layout settings, enter the path of the attached file.

Some guides will suggest only starting Skyrim through SKSE64 as a way to prevent the game from auto-updating on launch, but since SKSE64 is classified as a mod, that's not allowed. You'll have to switch Steam to Offline Mode whenever you want to launch.