Nope, you can't change a booking just cause the price you charged isn't to your liking. As per the other host responses.
FYI
I don't use smart pricing, and even then, I've accidently taken bookings at normal, not peak rates for weekends that should have been priced higher. It's my fault. (I know now when i change my default weekend rate, it overrides any special weekend that might command a higher rate).
Live and learn for next time
Kind regs MK