Answer:
I would politely reach out to XYZ organization and let them know I won't be taking them on their offer
Explanation:
In this particular situation, there is no legal binding in this agreement to start work. There is no responsibility from me to XYZ corporation.
If this new offer excited me more than what XYZ corporation has offered, then I have to decide which company i would decide to do my internship with. So I would have to politely decline the offer from XYZ.