The original owner of the Sesame needs to remove the connection between the Sesame and his phone. Here's a tutorial for how to do so: How do I remove/unregister Sesame from my app?
After the Sesame has been unregistered, manually reset Sesame and it will allow for a new pairing: Remove the cover by pushing down on the center of your Sesame, then press the reset button at the back for at least 5 seconds. Here is a tutorial that may help: How do I reset my Sesame manually?
If the owner has not unregistered the Sesame, resetting Sesame will not help. This is a safety feature to protect the owner of the Sesame. Please ask the original owner to unregister the Sesame via the Sesame app so you can register the Sesame to your own account. We are unable to assist if Sesame was not purchased from candyhouse.co or an authorized retailer. We apologize for the inconvenience! - CANDY HOUSE