Purchased older apple iPhone

I recently purchase an iPhone 5s from an older lady at a garage sale. when i factory reset the phone i found it is still registered to an account. I went back to the lady who is elderly and asked if she had could help by removing the phone from her account. she had no idea what i was talking about and gave me a receipt and contact phone number to use if needed. can any body help me .I have just purchased a new iPhone SE, my fifth iPhone and would love to be able to use the 5S for a second Phone.