After update iOS 8.4 3G is not working

Hello there, Nilesh.

It seems that after you upgraded to 8.4, you are not able to connect to your carrier's 3G network. The following Knowledge Base article offers up some great recommendations for troubleshooting:

If you can't connect to a cellular network or cellular data - Apple Support

If you see No Service or Searching on your device, or can't connect to a cellular network or cellular data, follow these steps:

1. Check your coverage area to make sure you're in an area with cellular network coverage.
   • If you can't connect to a cellular network, make sure cellular data is on in Settings > Cellular.
   • If you're traveling internationally, make sure your device is set up for data roaming. Go to Settings > Cellular > Data Roaming.
     
2. Restart your iPhone or iPad.
3. Check for a Carrier Settings Update.
4. Remove the SIM card then reinsert it. If the SIM card is damaged, worn, or bent, or doesn't fit in the SIM tray, contact your carrier.
5. Try another location. If possible, see if other devices are able to connect to your carrier's network in that location.
6. Contact your carrier to:
   • Verify that your account is active and in good standing.
   • Make sure there are no outages in your area.
   • Check that your device is not blocked from receiving cellular services and set up with the right data plan.
7. Update your iPhone or iPad to the latest version of iOS.

If you still can't connect to a cellular network, and have confirmed with your carrier that there isn't an issue with your account or network, try restoring your device.

Thanks for reaching out to Apple Support Communities.

Cheers.