that's a really open ended question.. we need to know a tick more info on what it is that you're trying to do.. example.. it's self extracting exe (zip/rar file turned into a standalone so that no third party app is needed to open it) or you're wanting to adjust a program's appearance then you're looking for a resource editor of sorts and a good one but a bit complex version is reshacker and it is free. Then there's Hex editors and other disassemblers that are for more complicated things persay.
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