You'll need to be more specific. I'm guessing your trying to update your firmware.
Are you using Odin?
What firmware?
did it come with all the files?
You might have a All-In-One firmware update which includes Modem Driver, which you will not see in the Modem Driver location in Odin.
Do NOT try and update your phone unless you are sure you have all the correct files, and are following the correct install procedure for these files.