3G iphone on 4.0 jailbroken, is there easy way to upgrade

[rusty]

hi all
been looking around but just keep getting confused
my lad has an old iphone 3G, which i jailbroke at christmas on to iso 4.0
been downloading a few games that won't run on iso 4.0
is there an easy way to up grade the iso, or do i have to re-jailbreak to the higher iso

[fanni]

you'll need to update it in itunes and then jailbreak it again 4.2.1 is the highest a 3g can go