My assumption is that you are looking at the "View Source" screen display.
You cannot modify this screen and re-execute the script.
You can however copy the source, paste it to a new textfile with NotePad (or other text editor)
save the modified code and then execute it from your browser's 'open file' command.
That also assumes there are not any external files being accessed.
If there are, they need to be copied as well if they are not referenced with a complete URL.