I have a data frame called School_Behavior.
I have a column "sex" with responses as 
- 1 for male,
- 2 for female.
How do I only change the 1's and 2's in this column?
I have a data frame called School_Behavior.
I have a column "sex" with responses as 
How do I only change the 1's and 2's in this column?
We can use do that with indexing.  Assuming that the 'sex' is character column, convert it to numeric, use that as index to pass the string vector for replacement
df1$Sex <- c("Male", "Female")[as.numeric(df1$Sex)]
df1 <- data.frame(Sex = c("1", "2", "1", "2", "2"), stringsAsFactors = FALSE)
