How to enter a new ID on an iphone that apparently already has one?

Hi everyone, i bought an iPhone 4 at a storage Who sells all kinds of brand. The seller calls it an “reconditionned” phone. which apparently means that the phone was used and is now repaired or restored. So the problem starts at the real beginning of the setting. I choose the language and country, I connect to the wifi and enter my sim card password perfectly. But, It is written that apple ID is already entered and if I want to enter a new one I need to contact the last user. How can I fix this ? Please