Hello ayad;
The operation you want to do is called an "Upload" in Siemens' environment.
Specific details for the procedure will depend on whether you are working with a S7300/S7-400 PLC, or a S7-200 PLC; two very different machines from Siemens.

For the S7-300/400 I recommend you read the chapter "Downloading and Uploading" in the "Programming with Step 7" manual; for the S7-200, the S7-200 system manual has a chapter named "PLC concepts" that explains the uploading procedure.

For S7-300/400 PLCs you will need the programming software Step 7 , with an MPI cable to connect the PC to the PLC,such as the PC Adapter (RS-232 to MPI converter).There are other hardware connections available, deopending on the configuration of your PC and PLC. You will also need to create a connection between the PC and the PLC; that is explained in the manual I suggest above, in a chapter called "Establishing an Online Connection".
For S7-200 PLCs you will program with Step 7 MicroWin; there is a PC-PPI cable to connect the PC to the programming port of the PLC; look for the section "connecting the CPU" in the Getting Started chapter of the S7-200 manual.

You must understand that when uploading from a CPU, if you do not have an existing version of the project, you will access the configuration and the logic of the program, but symbols and comments will not be available, since they are never downloaded to the PLC in the first place; they exist only in the PC program. So it will be very confusing trying to follow the program logic.

Hope this helps,
Daniel Chartier