Unfortunately I don't think you can. Because some of the characters used in the code of the Scratch project cannot be displayed by the editor, it displays a symbol instead. When you save that symbol replaces what was once there and you get a "bad header" message from Scratch when you try to open the project.
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Pretty much what sparks said.
Of course, in special cases, you may edit scripts in a byte editor such as Visual Studio, but it's difficult. Whatever you do, however, always make backups!
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