You'll need access to PostGIS source code, which is freely available on their site along with their binaries. Then pretty much install PostGIS again after patching the code.
The patching instructions is in the thread:
tar xvfz postgis-1.5.0.tar.gz && patch -p0 < postgis-1.5.0-RafalMagda.patch
Then compile and install PostGIS again.
If you're on a unix environment, this is quite routine and simple.
You can also do this on Windows, but will require additional software.
In step 15, you'll need to find and use a patch command similar to above.