First, switch to the version of HHVM that the documentation site is currently
targeting - you can check composer.json to check which this is.
This is needed as the scripts to update the versions are written in Hack.
To use the latest version of HHVM and the HSL:
$ bin/update-versions
The script will print out next steps; if you are changing the targeted HHVM version, this will include upgrading your local HHVM.
To use specific tags:
$ bin/update-versions --hhvm-tag=HHVM-a.b.c --hsl-tag=vx.y.z
See bin/update-versions --help for more options.