Kindly help me to update my iPhone 6

To update to the most recent, compatible version of iOS 12.5.7, please see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information. 

If you have already installed iOS 12.5.7, there are no more updates available for the iPhone 6. You have reached the end of the road with the iPhone 6.