A few min to shkieh before Shabbos standing next to my fridge I said, “I am mekabel Shabbos” and תוך כדי דיבור I realized that I forgot to switch fridge to Shabbos mode, so I said, “I take back my “קבלה. Does it work?
If it was תוך כדי דיבור it does work according to most poskim. The reason for this is because according to most poskim when a person is mekabel Shabbos, he can not be matir neder on it, because it isn’t considered a neder. That being the case, like any other statement, if one was regrets what he said תוך כדי דיבור it is nullified.
Eliyahu Rabba 263-38, Taz ibid -3, Aruch Hashulchan ibid 28, M:B ibid 31, Igros Moshe O:CH 3-38. See Kol Nidrei chap 70 ftnt. 2, and 11-8. Conversation with R’ Stesman shlit’a, author of Kol Nidrei. If it would be considered a neder, then it might be a neder l’dvar mitzva, and there are those who question if it is possible to back out תוך כדי דיבור, however since the basic halacha is that it isn’t considered a neder therefore, it is fine. If you would like to be machmir and be matir neder to fulfill according to the opinions that it is a neder you can.