Sorry, the current Sothink SWF Decompiler does not support editing ActionScript directly within the program.
The AS Class Structure will be listed in the Export Panel like a folder-tree according to its path in package. You can view ActionScript in the Preview Window, copy it to your clipboard and export AS resources separately. But you are not able to edit it.
However, after you export the SWF as FLA, you can edit ActionScript in Adobe Flash. The FLA file will updated automatically after the change is saved.