I´m tryinging to uppgrade my Iphone 4gs but get error 21

http://support.apple.com/kb/TS3694#error21

If after trying Lawrence's advice, you are still stuck with Error 21, try DFU Mode Restore. Here are the steps. Open iTunes on computer. Connect iPhone to computer with USB cable. Hold both Home and Power buttons, after exactly 10 seconds release the Power Button, keep holding the Home button until iTunes recognizes iPhone in Recovery Mode, usually about 20 seconds time. If the iPhone screen is black, blank this is DFU Mode. Click the Restore button of iTunes. DFU Mode can be difficult to enter sometimes, just keep trying. DFU Mode is a black, blank screen with iTunes saying Recovery Mode.

Frökullen wrote:

I´m tryinging to uppgrade my Iphone 4gs

Well, there is no such device.

Your signature shows 3GS.

What do you really have?

Identifying iPhone models

Look 2 messages above yours for a link to Error 21 help.

There are 3 common causes of error 21:

1. Your phone is broken.

2. Your antivirus is interfering with the update.

3. Your phone is jailbroken.

It's one of these 3. If you've eliminated 2 and 3 then your phone is broken.