Replacing values in one variable by giving condition on other

Suppose my dataframe has two variables x and y

x = c("A", "B", "C", "D", "E", "F", "G", "H", "I")

y = c("M", "M", "N", "T", "M", "M", "N", "T", "O")

df = data.frame(x,y)

df

x y

1 A M

2 B M

3 C N

4 D T

5 E M

6 F M

7 G N

8 H T

9 I O

I want to replace values in variable x where df$y == "M" and change those values to "K".