Please click the following link to download ns-3.31, released June 27, 2020:

For users of the ns-3.30.1 release, a patch is available to update code to ns-3.31, at this link. Other patches to migrate older versions of ns-3 (back to ns-3.17) to subsequent release versions can be found in the same directory; they must be applied sequentially to upgrade across multiple releases.

The sha1sum of the ns-3.31 release file is e7cce38ce4f81b62cbabce4990c4e1ede5daa552.