Some clarifications, you are not working with a csv file (on disk), you have loaded data from a csv file into a data frame, which is an object in memory that contains tabular data, so Big6 would be a variable or a column of your data frame and in this context "defining" a variable is a synonymous of "creating" a variable.
See this example
# This represents the data frame you are working with
df <- data.frame(
big6 = c(3, 3, 6, 5, 9)
)
df
#> big6
#> 1 3
#> 2 3
#> 3 6
#> 4 5
#> 5 9
# Now I'm going to define a new variable called new_recoded
# based on big6 and recode 3 as 0
df$new_recoded = df$big6
df$new_recoded[df$new_recoded == 3] <- 0
df
#> big6 new_recoded
#> 1 3 0
#> 2 3 0
#> 3 6 6
#> 4 5 5
#> 5 9 9
Created on 2019-11-06 by the reprex package (v0.3.0.9000)
There are better methods to accomplish this task but I suppose you have to use base R for your course