Update custom library
Text from here says this:


  In Arduino v16 and earlier, libraries were stored in the
  ArduinoInstallDirectory/hardware/libraries folder, which also
  contained all the built-in libraries (like Wire and Serial).
  
  In v17 and up, the user libraries are now stored in the
  ArduinoSketchDirectory/libraries folder.


Check your version of the Arduino SDK and then look in the appropriate directory for your library source code.  Then change it there.

The Arduino SDK is fairly simple and often compiles everything to avoid problems like this.  The installing from a zip file feature (unless it has changed) is more for convenience when downloading a single zip file.  I have often developed &amp; edited custom library source code where the SDK expects to find it.