go to Edit > Find and Replace… There’s a drop-down that will change everything in the single document, all open documents, all documents in a folder, selected files in a site or all files in a local site.
If your version does not have this then download Replace Text from This program can change multiple instances of code in multiple files.
If the header is an image, then you need to use an external CSS file and link to it on all your pages. That way, when you change the image, you edit just the CSS and the change appears on all pages.
If it is text or text and image, then you should use Server Side Includes (SSI). Then you just edit the one include file and all changes appear on every page that has the include code.
You will need a web host compatible with dreamweaver.
Take a look at:
Hope this helps.
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