Yeah essentially you just set the correct url with your /forum path in config.json and set your reverse proxy to forward that path onto NodeBB:NginX - NodeBB Documentation
Installing an older NodeBB Version
How do you install this version of Nodebb instead of the current one. The Background looks like this:
We can help more with a detailed example BUT....
Let's say you want the latest in the 0.6.x family. you would do:
git fetch; git checkout v0.6.x; git pull; ./nodebb setup
Or for a fresh installation:
git clone -b v0.6.x https://github.com/NodeBB/NodeBB.git nodebb