Help adding a refurbished iphone 6

Is it asking for the old owner's Apple ID password, or screen unlock passcode? If it's the passcode you need to restore the phone using iTunes: If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support

If it is the old owner's Apple ID password the phone is Activation Locked, and you must get the previous owner to follow these instructions: What to do before selling or giving away your iPhone, iPad, or iPod touch. Without the old owner doing this there is no way that you will be able to use the phone.

I had an Iphone 4. The battery was a problem for these phones; Apple replaced the battery without cost to the purchaser. In the meantime I purchased an IPhone 6. I wanted to give the Iphone 4 to my sister. I asked at a store here where I live (we do not have an Apple store) what to do. The attendant at the store unlocked my phone for $50.00 CDN.

My sister is happy to have an IPhone.