Unless you are already running a forked version of mojoPortal you should just use the latest code and upgrade your site using a build of the latest code. My advice is don't fork the code, if you didn't fork the code then there is no reason to use an older version of the code, instead you should build the latest code and deploy it to upgrade your site then if you want to clone the blog to make a custom feature you still could but you would be using the latest version. If you really need an older version of the source code then you need to review the changesets in the repository to find the revision that is tagged as version 2.3.8.5. Then you use Tortoise HG > Update, in the dialog you specify the revision to update your working copy to that revision.
But again, it is crazy to use such an old version unless you already forked the code and shot yourself in the foot.