Jan's advice is very wise.

Disassemble your own hex file. Identify your most common functions and edit the labels to your original names.

You will discover how to identify code blocks, data blocks and the general structure of your program. Edit the disassembler control file to recognise these different blocks.

Your disassembled code gradually becomes readable. Repeat the exercise with a hex file generated by one of your own C programs. You should be able to edit the source, move stuff around a bit and still get a working binary program.

You will learn which Compiler has produced which hex file.

Now you can start with a foreign hex file. It is hard work but possible to re-create a working source code. But as others have suggested it far easier to just write the whole thing from scratch.

David.