How can I upgrade my icloud storage on iphone 6plus

u need to go to settings look for your name up at the top click on it, find icloud on that page. After that you would se details about your iCloud storage on next page and there should be an option to manage storage just click on that and change storage plan

are you getting any error also please make sure your device has up to date software. Usually it is because of software issues If you still not bale to i would suggest to buy it from any other ios/macos/ iPados devices using your apple id.