you need source code for that , kung wala kang source then know what language it is written, kung compiled siya into machine code, halimbawa eh java, use decompiler pero not 100% okay na agad since napaka-complicated ng compilation, madalas hindi na siya tulad sa source pero nandoon pa rin 'yung function pieces, good luck na lang sa "arbitrary" variable names, example:
// original
int height;
int width;
// decompiler (cfr)
int a1;
int a2;
pero kung ang language eh non-compiled , o rather compiled at a point of execution, gaya ni python, then mas mapapadali ang buhay hehe.