How do I delete previous owners Apple ID and iTunes ID so I can replace it with my own
How do I delete the previous owners iTunes and Apple ID and iCloud ID so I can replace it with my own
iPhone 5s, iOS 8.3
Your device is Activation Locked as explained here:
Find My iPhone Activation Lock - Apple Support
See "What if I purchase a device that is still linked to the previous owner's account?" in the article.