|Although the link posted by Razor will give you a dissembler that will generate assembler code, to make use of it you would need to know assembler instructions and if you knew that you wouldn't be asking the question in the first place. You would also need a compiler to reassemble the code into an Exe.|
What you really need is a decompiler which will attempt to generate the source code that was used generate the EXE in the first place. Even then it wont be exact but will give you something to work with. Which decompiler you use is dependant on which language was used to write the EXE in the first place. You can usually work out which programming language was used to write the application by looking at the EXE with a Hex editor but you have have some familiarity with programming to work it out.
In short, unless you have some programming experience. modifying an EXE is a daunting task. A bit more complicated than altering a few lines in a batch file.