Update: There is an alternate procedure now available, using a binary patcher to downgrade an existing Steam installation (English only) of Skyrim version 1.6.318 back to 1.5.97. On Nexus: Unofficial Skyrim Special Edition Downgrade Patcher. I haven't tried it so can't vouch for it, but it's easier to use and smaller to download than the following. The following procedure allows you to download from Steam and install a pristine vanilla copy of the last Skyrim SE version (1.5.97) before
