I have a SIMANTIC MANAGER software that runs for SIEMENS S7 PLC. Now I come across a S5-100U. Can I use the same software and I need another software version to edit the PLC program?
You cannot use STEP7 to program an S5.
You will need at least "STEP5 MINI" from Siemens or "S5 for Windows" from IBH.
The latter is probably the easiest to get into.