You possibly mean you need to convert a hex file (like S19) to ASM. Assuming the file actually contains 9S08 code, you would need a disassembler, preferably a code-seeking one. Although this tool can help by doing some of the job, it cannot do a complete reversal of the ASM file (except for trivial cases) without significant operator input.
Don't forget a lot of the source-code information is irreversibly lost when translating from ASM to machine code, and this needs a lot of human involvement to attempt to recover -- and, then again, with no guarantees of success in all cases. I would say this may be a daunting task, and in most cases it turns out a much more difficult one than simply re-writing the app's functionality from scratch.
If, however, you have no choice but to actually recover this specific file (e.g., to recover perhaps some very valuable algorithms,) you'll need to disassemble with the help of some tool, and a *lot* of manual work (proportional to the size of the application).
Another possibility to using a disassembler is to use a software simulator in which you load your app and execute it until you have managed 100% branch coverage. This will reveal your full code little by little. Again, a lot of manual work will be required.
Good luck.
