Trying to correct values using mutate case_when and it's not changing any values?

I have 4 csv files representing 4 different nests. I'm trying to combine them all into one spreadsheet. I have a few errors that were made when inputting data and I am trying to change them in R. For example, "GBGB" is written instead of "GBBG" for nest 4. This is the code I used:

nest.4 <- nest.4 %>% 
  mutate(Species = case_when(
    Species %in% c("GBGB") ~ "GBBG"))

but it didn't change any of my values! I also want to combine them all at the end once I fix these errors. I want to use :

nests.df <- left_join(nest.1, nest.2, nest.3, nest.4)

but I am getting this error message: Error: by must be a (named) character vector, list, or NULL for natural joins

I'm just learning how to use R so I don't know if this is the right way to change values and combine datasets or not! Thanks!