how can i use icloud backup if i am the third party owner on this iphone,it wasnt my name on the icloud apps only itunes was on my name

Here is the device-transfer procedure, which is own ownership and activation lock is transferred:

What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

With your Apple ID, iCloud access, and the rest are all associated with you.

An iPhone 5c (introduced in 2013) cannot be upgraded past iOS 10, as it lacks the hardware and particularly the memory necessary for newer iOS versions. There is no hardware upgrade available. The only option for more recent iOS versions is a newer or new iPhone.