I cannot update my iphone 6 plus to 10.2.1 its always saying error

Hello,

If you can’t reach the update server or can’t verify the update

If you try to update the iOS on your device, you might see one of these messages:

"Unable to Check for Update. An error occurred checking for a software update."

"Unable to Verify Update. iOS failed verification because you are no longer connected to the Internet."

Try updating your device again using the same network. If you still see one of these messages, try updating your device using another network. If the issue happens again after trying to update with multiple networks, remove the update.

Learn more about iOS updates.

More info here:

Get help with over-the-air iOS updates - Apple Support

Hi!

Are you using iTunes or are you downloading it from your iPhone?

Have a look here: Update the iOS software on your iPhone, iPad, or iPod touch - Apple Support

Have a look here to find solutions for errors: Get help with iOS update and restore errors - Apple Support

Bye

Samuele