My iPhone 3gs got an update today ios 6.1.6 but when i press install update my phone shutdown and shows connect to itunes display, after i connect to itunes it shows unknown erro (306).....whats is happening and what should i do ?

Take a look at this post: https://discussions.apple.com/thread/5036467?tstart=0

iTunes: Advanced iTunes Store troubleshooting

http://support.apple.com/kb/TS3297

iTunes Store loads partially or returns "Error 306" or "Error 10054"

Proxies, parental control settings and software, security or filtering software, or a bad iTunes Store cache can cause this.

To address proxies, Remove Internet Options proxy settings and connect to the Internet without a proxy.

To reset iTunes Store cache:

1. In iTunes, choose iTunes > Preferences (Mac) or Edit > Preferences(PC).
2. Click the Advanced tab.
3. Click the "Reset cache" button.
4. Click OK and see if the issue is resolved.

Adjust Parental Controls in iTunes:

1. Open iTunes.
2. Access iTunes preferences:
   1. On a Mac: From the iTunes menu, choose Preferences.
   2. On a Windows PC: From the Edit menu, choose Preferences.
3. Click the Parental Controls tab.
4. Remove restrictions on Parental Controls.

For more information on parental controls or content filtering software, seeiTunes 10.5 for Windows: May see performance issues and blank iTunes Store.

For more information on other security software, see iTunes: Troubleshooting security software issues.

This page also deals with this error.