yeah I found out.After changing them with my tool or a cheat? You can't. But you can force the game to generate a new set of prices by time-traveling any amount of time, even a single minute, into the past. Be warned that it will spoil all turnips, and you will also need to water your flowers again.
(You can also time-travel forward to the next Sunday or later, but traveling a couple minutes to the past has less side-effects.)