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Assuming you have already downgraded your Skyrim SE to 1.5.97, here is how to install the correct USSEP. Unofficial Skyrim Special Edition Patch 1.5 97 Download
For players using (often called the "Pre-Anniversary Edition" or "SSE" version), finding the correct Unofficial Skyrim Special Edition Patch (USSEP) can be tricky because the official mod page typically only hosts the version for the latest game update. The Correct USSEP Version : Assuming you have already downgraded your Skyrim SE to 1
What it fixes:
Many players use the to revert their game engine back to version 1.5.97. This allows them to run older, highly stable mod setups. This allows them to run older, highly stable mod setups
Skyrim Special Edition (SSE) version 1.5.97 is widely considered the "golden era" version for modding. It represents the final stable build of the game before Bethesda introduced the Anniversary Edition (AE) update (version 1.6+).
If you have downgraded to 1.5.97, you use the latest USSEP (which is built for game version 1.6.1130 or higher). You need the archival version: USSEP 1.5.97 .