How do I update my iphone from version4.2.1

You don't.

Iphone 3g can only handle up to 4.2.1

Dear Mr. Smith,

I accidently removed whatsapp from Iphone 3G version 4.2.1.

Can you help us to install it again?

The official Apple Helpdesk told us we need version 4.3, but we hope

you know a solution to our problem. That would meen a lot to have again

the possiblitity to use Whatsapp.

Thank you,

Mr. and Mrs. Hoving