> test <- data.frame(name = rep(letters[1:3], each = 3), var1 = rep('a', 9), var2 = rep('x', 9))
> test
  name var1 var2
1    a    a    x
2    a    a    x
3    a    a    x
4    b    a    x
5    b    a    x
6    b    a    x
7    c    a    x
8    c    a    x
9    c    a    x
> str(test)
'data.frame':   9 obs. of  3 variables:
 $ name: Factor w/ 3 levels "a","b","c": 1 1 1 2 2 2 3 3 3
 $ var1: Factor w/ 1 level "a": 1 1 1 1 1 1 1 1 1
 $ var2: Factor w/ 1 level "x": 1 1 1 1 1 1 1 1 1
>

i am going to replace all occurrences of pattern "a" with pattern "ABC"
of data frame "test"

test[] <- lapply(test, gsub, pattern = "a", replacement = "ABC", fixed = TRUE)


> test[] <- lapply(test, gsub, pattern = "a", replacement = "ABC", fixed = TRUE)
> test
  name var1 var2
1  ABC  ABC    x
2  ABC  ABC    x
3  ABC  ABC    x
4    b  ABC    x
5    b  ABC    x
6    b  ABC    x
7    c  ABC    x
8    c  ABC    x
9    c  ABC    x
> str(test)
'data.frame':   9 obs. of  3 variables:
 $ name: chr  "ABC" "ABC" "ABC" "b" ...
 $ var1: chr  "ABC" "ABC" "ABC" "ABC" ...
 $ var2: chr  "x" "x" "x" "x" ...
>