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...or could just use one of the many free HTML editing softwares available. Or even download the files, edit and reload. Maybe even use the file editor form the server control panel...
This is a feature request not a 'How to'!
Other Web based softwares have a feature that will access a file on the server, show the code and allow editing and saving using the browser. This is within the program admin. XOOPS only let you see the code but you can't save any changes