We need a bit more information to be able to help.
What is the model number of the AirPort Express?
If you do not know, look on the side or bottom of the device. The model number starts with an "A" folllowed by four numbers. Something like A1264, for example.
What operating system are you using on your computer?
If you have a Mac, click the Apple icon in the upper left corner of the screen and then click About This Mac. Post back with the OS X Version number that you see there.
Finally, it would help if we understood where you are getting stuck when you try to update your Apple router. Please provide the steps that you are using....and what happens after each step.