You need a "disassembler", a program that reverse engineers the bytes back to their corresponding instructions. If your file is in .hex format (a text file) you might have to convert it back to binary first, some disassemblers will do this for you.
What it will not do is reproduce the source code as it was written because the structure, variable names and comments are not saved in the HEX file but it will give you a list of instructions you have to interpret yourself.
There are disassembler programs for most devices but as each produces it's own bytes according to the processor type, you need one specific to the device you are using.
Brian.