Unable to update iOS 9.x version to 12.x version

I have a 64GB iphone 6 and would like to install iOS 12 since i didn't update my OS for a long time. My iphone is on 9.3.5 and i couldnt see the option for update the iOS directly from my phone. I tried to install using itunes but i'm getting the message "This version of the iphone software (9.3.5) is the current version" when i click update button on the summary panel. Please help me to resolve this issue