You could nuke the whole thing and start over, by just following Andrew’s link, that would always be the safe thing to do. I suppose your question is: Do I have to do that or is there a safe way to just get the updates from the branch and building again?
I think the following is pretty safe (I did it a couple of days ago myself):
cd Rack
make clean
cd dep
make clean
cd ..
git checkout v0.6
git pull
git submodule update --init --recursive
make dep
make
On Windows and Linux you can add -j$(nproc) right after make for performance, and on Mac it’s -j$(sysctl -n hw.ncpu) instead.
Yeah, when I upgrade on Win/Linux I nuke it. But I have limited time on the Mac, so I prefer to just update the branch there once I get access to it again.