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Can this help?
What software do you use to program your website? Is it only local (offline= or is it also available online (if so, which hosting service)?


It's only local. I'm keeping it that way until i get trademarks and until it's finished.
As for programming I use TextWrangler.

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I'm working on a website right now its only local but i want to edit the page not found page.

404 page is server configuration set, while you can create a page that you want to use, unless you have access to the web server config you can't default the page you create to be the default 404 page. Alternatively discuss with your ISP if that's a setting you're allowed to configure. simsaladimbamba gave you a good link for an Apache host, an IIS host does not follow the same configuration.

Here's a general overview on an IIS server: http://ask.enquiro.com/2008/how-to-create-a-custom-404-error-page-in-microsoft-iis/

Then there's also the CustomErrors options in .NET...
 
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