PLC AC500 ETHERNET CONNECT?
Use a crossover cable when connecting directly to the PLC. I assume you have tried the default PLC ip adress ?
I would strongly advise that you do NOT change the PLC address unless you absolutely know that this is safe to do. In any case, you need to know the existing address before you could change it.
You need a copy of the application software currently running in the PLC. Hopefully a backup of the project is stored on the XD card inserted on the front of the PLC. Insert this into your PC and read the backup.
Once you have a copy of the PLC project you can see the IP address of the PLC and set your PC to the same subnet range.