It doesn't.
The '.pwn' file is the human readable script (source). The file extension doesn't even matter (although do use it, or any of the valid pawn (the language) extensions: .p, .pawn and .pwn (although .pwn will be fully valid per compiler version 3.10.11:
https://github.com/pawn-lang/compiler/pull/701) and .inc for includes).
The server can't read those human readable files. It needs the compiled file, an .amx file (which is the file you get when compiling the source file).
Since this doesn't relate to your initial question (how to edit .dll files), you probably meant those .amx files. And so the answer to that would be: You can't: edit your source file and then compile it.