This is easy. Go. Because you said you would. But have a good time. She isn't your girlfriend, so you're not breaking up with her. You aren't even dating. There is nothing to worry about at this point, you owe her nothing (other than your time you said you would give). She is just a girl that you are going out with for the day.
For the date. Do something that you want to do, have a good time and at the end tell her it was nice to hang out, don't hold her hand, don't give her a hug, etc. But also, don't act bored, give her your attention and think of this as practice for future dates with girls you do like.
Then afterwards go about your business as you would normally. Don't text or call and after a few days she should get the idea. If she texts or calls and wants to know what's up, tell her you had a great time and she seems like a cool friend.