If you want to overwrite the existing values in the dataframe:
data %>%
mutate_at(vars(variable_here), ~log(.))
(I should note that the mutate_at() approach will be soft-deprecated with dplyr 1.0.0)
And if you want to create a new variable in the dataframe holding the logged values:
data %>%
mutate(log_var = log(variable_here))