how to a cancel or change a date?

Get the girl's number, call her, and ask what she wants to do today. Offer up a specific plan - get coffee or a drink, for example. That's a low-key activity so you can get to know each other better.

If you are unavailable today, call her and say, "Hey Susie, I know Manager set up a date for us today, but to be honest I was caught off-guard and didn't think it through before committing. I'd still like to go out with you, though - next week actually works better for me. Would you like to get dinner next Friday night and maybe see a movie? Could I pick you up around 6 pm?"

If you work with her, you can also talk to her today at your lunch break or something, and see if she wants to go out tonight or next week.

Next time, don't let your manager handle your love life. If you and the girl agreed that Manager could set you up, then you should've said, "OK, give me her number and I will call her and set the plans myself."

just text her and tell her today is not good, ask her is shes not busy can yall change it to _____(insert the date and TIME!)

Why the hell is your manager setting you up on a date with a coworker? Seems inappropriate. Say to the coworker (leave your manager out of it) that today is no good, and ask if you guys can reschedule it for next week instead. Say something like, "Hey I am really looking forward to our date. But sadly, tonight is no good for me. Are you available on a day next week to do it instead?"