I've not come across .bin files being used for PIC stuff. The Microchip tools produce .hex files for burning to the chip. Still, it's probably the same thing.
Assuming it is...
You've got a wonderful world awaiting you! You have to disassemble the file to get back to mnemonics. Since the code will have lost any identifiers (like comments, address labels, or constant names, or other useful stuff the compiler lets you do) you're stuck with a load of hex numbers. You'll have to work out from the code itself what the variable names should be, what the sections are, what the look-up tables mean, etc etc etc. It's not *impossible* to reconstruct source code like this, but it's tedious and time-consuming.
What does the code do? What does it need to do once modified? Honestly, you might find it quicker and easier to start from scratch if it's not something super-sophisticated.
