If the compiled AppleScript, or applet, was not saved as a Run-Only application, there are several ways to modify it. An AppleScript that has been saved as Run-Only does not save the original AppleScript text for re-editing.
Here are three ways to access the AppleScript Text of a non Run-Only applet.
1) Press and hold the control key while double-clicking on the applet. When the Applet's splash screen appears with its Run or Quit options, select Edit Script from the Edit menu. This will launch the Script Editor, if it's not already running, and open a Script Editor window with the compiled AppleScript text.
2) Drag the AppleScript Applet onto the application icon of the Script Editor. This will launch the Script Editor, if it's not already running, and open a Script Editor window with the compiled AppleScript text.
3) Launch the Script Editor and choose Open Script from the File menu. Use the open file dialog to navigate and select the applet you wish to edit. The Script Editor will open a window with the compiled AppleScript text.