data(iris)

put_1 <- function(x){ x = 1}

iris %>%
   mutate_at(vars(Petal.Length, Petal.Width), funs(put_1)) %>%
   head()
#   Sepal.Length Sepal.Width Petal.Length Petal.Width Species
# 1          5.1         3.5            1           1  setosa
# 2          4.9         3.0            1           1  setosa
# 3          4.7         3.2            1           1  setosa
# 4          4.6         3.1            1           1  setosa
# 5          5.0         3.6            1           1  setosa
# 6          5.4         3.9            1           1  setosa