To have a reasonable chance to convert the original program, then you must have either a printout with comments and symbols, or the project file so that you can create a new printout.
If that is OK, the next requirement is a minimal understanding of S5.
If you are very lucky then you can view the program in LAD (ladder - aka KOP), if you are almost as lucky then you can view it in FBD (function blocks - aka FUP), if you are out of luck then you can only view it in STL (statement list - aka AWL).
Converting from LAD should be straightforward, from FBD not so difficult, from STL very difficult.
There IS a possibility to protect program blocks on the S5 system, but it is rarely used.