3.6.13 hasn't been released yet (in fact it never will - the release will be 3.6.14, 3.6.13 is just the numbering for the development branch) which is why there's no single download, and because the entire executable is one file, there's no way to build an update download. Because the executables are so small, a few MB, you don't really need to worry about download size anyway.
However, if you'd like to test 3.6.13, go to the Nightly Builds forum and download those. You can see a log of what's changed on each new nightly in the thread for the nightly (it'll also contain download links for that nightly.) Just be aware that the nightlies are more likely to have bugs.