I want to edit movies i buy, is this legal?

i purchased a movie from kmart, im not gonna say which movie but i want to remove some words but still keep the images and maybe fill it in with other words, for example when like you see on tv when in a movie you know they say the "F*** you" but replace it with something like "Fudge you" i wanna do this but want to know if its possible on the home computer, what software do i use and is it legal?