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Open /themes/themeFolderName/theme.html and edit changes. Or, upload an image of the same dientions to /themes/themeName/images (or whatever the theme specifies for a folder), refresh and done.
Dont forget to activate template updates in the admin panel or you will not see changes to your site.
You can also search here on the forums for troubleshooting and further tips on modifying your themes.
Also be sure to check out the links in my signature for more help if needed.
Good Luck.