this is a source code edit. Make a "building stuff" directory, and "apt source geany" (probably will need "apt build-dep geany" too). Edit the cxx file in there. From the top level of the source folder, you can run "./configure; make". After that, you should have an executable in src/geany.
Before that can run, it'll need either a "make install", or you could get away with symlinking /usr/local/share/geany to the distro's existing /usr/share/geany.