Since MH-E 8.6 was released in 2016, its development migrated to the Emacs repository. MH-E is now only supported in the version of Emacs in which it appears. Old releases of MH-E are still available for download at SourceForge.
After you download and extract the MH-E tarball, read the README file and MH-E-NEWS. These correspond to the release notes and change log mentioned above. The file README contains instructions on installing MH-E. If you’re already running Emacs, please quit that session and start again to load in the new MH-E. Check that you’re running the new version with the command M-x mh-version.
In addition to the mh-e package, the SourceForge site also contains doc and contrib packages. The former is the latest release of this manual, and the latter contains a few contributed packages you might find useful.