My iphone model a1203 and ios 3.1.3?

I need to upgrade my iOS is it possible what is the compatible iOS for the above model?

You did not mention your iphone version. Is that iphone 1, 2, 3, 3gs or what?

Https://discussions.apple.com/thread/3708027?start=0&tstart=0

Check out this apple forum!

Ah, an original iPhone. This could be quite a collectors items.
IOS 3.1.3 is the highest iOS for that device. You can't go higher.
Get any apps you need while you can. ITunes is nearly useless in searching for compatible apps.
You can try,
https://sites.google.com/site/appleclubfhs/support/advice-and-articles/finding-apps-for-older-devices
Or good app to help discover compatible apps right on your iPhone,
https://itunes.apple.com/...?mt=8&uo=4