I want to upgrade Subversion. How do I do that

You will first have to build Perl from scratch because Subversion support in Git is implemented in Perl, using the Perl/Subversion bindings. Most likely you will actually have to upgrade Perl first, something that is not for the faint of heart and would impress the Git for Windows maintainers no end.

After that, you will have to adjust /src/subversion/release.sh to download, build and install a new Subversions version. Most likely it will be similar to Perl: it won't work out of the box, so you will have to adjust/add patches to /src/subversion/patches/. The release.sh script will have unpacked the Subversion sources into a subdirectory and initialized it as a new Git repository to facilitate working with/on the patches.