To replace a character string in a column, use str_replace_all() and mutate().

# With the pipe operator
df <- df %>%
	mutate(col = str_replace_all(col, "string to replace", "replacement string"))

# Without the pipe operator
df <- mutate(df, col = str_replace_all(col, "string to replace", "replacement string"))

The code above will replace every single instance of string to replace found in col in the dataframe df with replacement string.

More specifically, str_replace_all() replace string to replace found in colwith replacement string and mutate() replace col with the version where string to replace has been replaced.

Documentation

Replace matched patterns in a string. — str_replace
Vectorised over string, pattern and replacement.
Create, modify, and delete columns — mutate
mutate() adds new variables and preserves existing ones;transmute() adds new variables and drops existing ones.New variables overwrite existing variables of the same name.Variables can be removed by setting their value to NULL.