Unfortunately Leila, a bad merge is one of the things that can't be undone.
I noticed that Priscilla's current Last Name At Birth (LNAB) is Rickard, which is the same as the LNAB for her father and for two of her sisters. I am assuming that you have evidence that her LNAB should be Richard, not Rickard. If this is the case, you can merge her with her sister Prscilla, who is a duplicate, but with the LNAB of Richard.