If the previous owner is with you a

If the previous owner is with you and can access the device
Ask them to erase all content and settings by choosing Settings > General > Reset > Erase All Content and Settings. They'll need to provide their Apple ID and password. After the device is erased, you'll can proceed with the device setup process.
If the previous owner is not present
Make sure the device is powered on and connected to a Wi-Fi or cellular network. Then contact the previous owner and ask them to follow these steps to remove the device from their account:
Sign in to their iCloud account at www.icloud.com/find.
Click All Devices to open a list of devices linked to their account, then select the device to be removed.
Click the Erase button to erase all content and settings from the device. When prompted, don't enter a phone number or message. Click Next until the device is erased.
When the erase is complete, click "Remove from Account" to remove the device from the account.
After the device is erased and removed from the account, you can proceed with the device setup process.