I'm not going to show up for something I told a friend I would.
Why wouldn't you show up for your friend?
I found something else I'd rather do.
Having second thoughts about a commitment is normal. That said, we should keep our word, or those we care about will learn they can't trust or rely on us. That damages relationships.
I'm gonna be a dick and say whatever I want.
Wanting to do something else is fine, but if you're going to cancel then you should let your friend know in advance. Or even just tell them that you would prefer to do something else and be upfront. Ultimately the wrong thing is not showing up.