After opening it as a palette, you can use Palette menu - Generate Notebook from Palette to put it in editable form.

You can find more information about making and installing palettes here:

http://reference.wolfram.com/mathematica/tutorial/CreatingPalettes.html

You typically find the current palettes under the two directories, one in the base directory and one in the user base directory. Evaluate the following code to see where these directories are on your computer:

ToFileName[{$UserBaseDirectory, "SystemFiles", "FrontEnd",  "Palettes"}]

ToFileName[{$BaseDirectory, "SystemFiles", "FrontEnd", "Palettes"}]