Hello,
how can I reverse engineer a file that is not executable, for example a firmware file.
Either in Windows or in Linux.

Thank you

Well, you are hiding every bit of information on your background.
I mean, we don't know how skilled you are, if you ever worked with a disassembler, if you know how to debug and so on...
Moreover you didn't tell what platform we are talking about - PC, some embedded device, Smartphone, whatever. You know, all these can run Win or Linux.
Since we lack all this information, I can give you just a basic advice. If you want to analyze the file statically go for IDA or another disassembler - if you want to do it dynamically just debug the executable that loads your firmware file.

Regards