If that is the only value you want to change you can simply do something like this
library(dplyr)
sample_df <- data.frame(
stringsAsFactors = FALSE,
subject = c("AAA", "BBB", "CCC", "DDD"),
Group = c(1, 2, 3, 4),
variable = c(1.5, 3.1, 3.9, 1.2)
)
sample_df %>%
mutate(Group = if_else(Group == 4, 3, Group))
#> subject Group variable
#> 1 AAA 1 1.5
#> 2 BBB 2 3.1
#> 3 CCC 3 3.9
#> 4 DDD 3 1.2
Created on 2020-02-05 by the reprex package (v0.3.0.9001)