$ opam upgrade cohttp
Everything as up-to-date as possible (run with --verbose to show unavailable upgrades).

The following packages are not being upgraded because the new versions conflict with other installed packages:
  - cohttp.6.0.0~beta2
However, you may "opam upgrade" these packages explicitly, which will ask permission to downgrade or uninstall the conflicting packages.
Nothing to do.

$ opam install cohttp.6.0.0~beta2
The following actions will be performed:
=== downgrade 3 packages
  ↘ conduit          7.1.0 to 6.2.3       [required by conduit-lwt]
  ↘ conduit-lwt      7.1.0 to 6.2.3       [required by cohttp-lwt-unix]
  ↘ conduit-lwt-unix 7.1.0 to 6.2.3       [required by cohttp-lwt-unix]
=== recompile 3 packages
  ↻ github           4.4.1                [uses cohttp]
  ↻ github-unix      4.4.1                [uses cohttp]
  ↻ opam-publish     2.4.0 (pinned)       [uses github]
=== upgrade 3 packages
  ↗ cohttp           5.3.1 to 6.0.0~beta2
  ↗ cohttp-lwt       5.3.0 to 6.0.0~beta2 [uses cohttp]
  ↗ cohttp-lwt-unix  5.3.0 to 6.0.0~beta2 [uses cohttp]
=== install 1 package
  ∗ http             6.0.0~beta2          [required by cohttp]

Proceed with ↘ 3 downgrades, ↻ 3 recompilations, ↗ 3 upgrades and ∗ 1 installation? [y/n] n