changed apple ID online but cannot use from iphone
changed my apple id online but the old id appears on my iPhone and I cannot login from my iphone
iPhone 6, iOS 8.4
You've completely logged out of the account on your phone, or are you just trying to download something and it's showing the previous version of your id ?
For the stores tap on your id in Settings > iTunes & App Store and log out of it and then log back in with the updated version of your account. And similarly for the cloud (Settings > iCloud > Sign Out) and log back in, and also check Messages and FaceTime
After changing your primary email address : What to do after you change your Apple ID email address or password - Apple Support