Grab the .7z file from the link above.
By staying on 1.6.640 and "locking" the version, you prevent Steam from automatically updating your game and breaking your entire mod list. Common Issues & Fixes skse 16640 link
The Data folder from the archive contains script source files. Merge this Data folder with your game’s Data folder. Use your mod manager (MO2 or Vortex) to install the Scripts.rar or do it manually. Grab the
( skse64_loader.exe , skse64_1_6_640.dll , and skse64_steam_loader.dll ) and the Data folder into your main Skyrim directory (usually Steam\steamapps\common\Skyrim Special Edition ). skse 16640 link
To download (Skyrim Script Extender) for Skyrim version 1.6.640