Sadly you can't.
An .exe file is made up of binary numbers which consist of 1's and 0's.
Operating systems use binary languages to operate, this is what we call machine code.
Whenever you right code. It's just a simplified version so we can understand it better.
Once the program is compiled it changes the code to binary (which the computer understands) and then it runs the code.
So there's no way to reverse this process. If there was a way to do so then anybody could alter with professional programs and operating systems.
If you learned the assembly language then you could probably learn to alter some things in the code but there's absolutely no way to change from binary to source code.
Also since binary consists of ones and zeros it doesn't remember the syntax. What a compiler does it take that languages format and changes it to what it would be if you were to write in binary.
So there's no way to detect which language you typed the .exe file in and that's one major reason why you cannot bring back up the source code.
So I Hope This Helped.
If you want the source code then you have to ask the people yourself, otherwise you won't be able to obtain the info.