Can't change my apple id on iphone 4s?

I want to change my apple id and i know that how can i by going to settings>itunes and appstores>then tap on the apple id but here i can't tap on it. It is light black so how can i fix it please tell me

Try rebooting it first. Hold power and home at the same time till you can turn it off > turn it off > hold power till you can turn it back on… It will take bit to boot back up.