Randomly replacing values in data frame

I would like to randomly replace values in my data frame, so that I would have 3 names with NA values, instead of having just one.

Names <- c("Tom", "Alex", "James", "John", "Wayne", "Harry")

a <- c(1,3,6,8, NA, 5)

b <- c(3,6,9,1, NA,1) 

c <- c(7,3,1,6,NA,6)

d <- c(1,4,7,2,NA,8)

data <- data.frame(Names,a,b,c,d)

I've managed to get this:

newdata <- data

for (j in 1){

  for(i in 2:5){
  newdata[j, i] <- NA
}}

But it replace values from the first row. Could anyone please help me and tell me how can I make it replace values in random rows?