Hello Nikunj,
Thanks for reaching out in Apple Support Communities. We understand that you're having trouble activating Touch ID on your iPhone 6 after updating your iPhone. Looks like you're getting an error message "Unable to activate Touch ID on this iPhone." We're happy to help. Have you check the Home button for any damage or debris? If your iPhone has a screen protector remove it and try to activate Touch ID again. We want to make sure nothing is causing interference with the Home button.
Check out the following support article:
If Touch ID isn't working on your iPhone or iPad
If the steps above don't help, reach out for additional support here: Get Support
Cheers!