HT4623 how do i update my iphone 3gs as it keeps saying that 4.2.1 is the right update which its not

Question

how do i update my iphone 3gs as its got 4.2.1 and its saying that it is the right updatee but its not

Answer 1

To expand on the 1/2-answer that Diesel gave;
The first thing you should look into is which version of iTunes you have.  If you're not running iTunes 10, then you're not going to be able to update your phone past iOS 4.2.
Depending on what kind of system you're using, there are several reasons why your iTunes may be out of date:
In the Mac world, if you're not running a version of OS/X newer than 10.5.x (Leopard) then iTunes won't upgrade to the latest version (10).  If you have a pre-2006 G-series processor Mac, you're pretty-much stuck at 10.4.x (Tiger) or lower and, sadly, Apple/iTunes doesn't support those machines any more.  If your Mac is newer than 2006 but older than 2008, you'll likely be stuck at finding a copy of Snow Leopard (10.6.x) and you need to see your local Apple service provider to see if they can get you updated.
If you're running a Windows workstation, I believe that any Windows machine that's still running XP patched to Service Pack 2 or later should still be able to install and use iTunes 10.  The system requirements can be found on the iTunes download page.  If for some reason your iTunes has stopped doing its updates (or won't do them) you'll probably need to re-install iTunes.  Installing over-top of the old iTunes should work and you won't loose any of your data - however, just in case, you should back-up your computer prior to making any significant changes to the software.
Now, Diesel also made the assumption that you may only have a 3G phone and not a 3Gs.  To verify this not the case, plus for anyone who may be reading this and has any doubt, this link will help you identify your phone.  iOS 5 and later does not support phones older than the 3Gs, and its unlikely that your cellular service provider will for very long either.
Good luck.