Erica Sadun about updating Xcode. The Mac App Store prompted her to update Xcode, and that's what she did. Unfortunately, the download is very big and you don't really have control over the download and update process.
I've learned long ago that it's much better to simply download the .dmg file by logging in as a developer, going to the Xcode downloads page, click on the link "Additional tools", and download the .dmg file from there.
I rename the existing installation to, for example /Applications/Xcode-7.2.app so I always can go back. And from the .dmg, copy the new version to /Applications. Then if my current project nicely compiles and deploys, I remove the old Xcode installation.
As an aside, I really hope Apple will give developers a way to download that multi-gigabyte file from the commandline or perhaps in chunks of 500 megs or something. The whole root of the XcodeGhost problem was that developers were downloading Xcode from a fast 3rd party server, because the usual route wasn't feasible.