Why can't I download Facebook back onto my iPhone 3G?

I deleted the Facebook app off my iPhone 3G last night and I'm trying to get it back but every time I go to the app store and try to get it back, it says "Update Required, This app requires iOS 4.3." My version on my phone is 4.2.1. (8C148). How do I upgrade to 4.3? Please someone help.

Because iphones are not that good… Go for android… Yo get so much variety…