As lotam said, if you’ve just changed the SIM, and it was working before the SIM change, then there should be no need to change firmware simply to update the APN details.
If your WMP100 has an embedded Open-AT application, you will need to consult the application provider or developer for instructions to update the APN details in that application. You should not change the firmware without first confirming with the application provider/developer!
If your WMP100 is controlled by some external application, you will need to consult the application provider or developer for instructions to update the APN details in that application. You should not change the firmware without first confirming with the application provider/developer!