There are many ways of doing it, but it's going to depend on the format of your dates, see this example.
# Sample data
df <- data.frame(stringsAsFactors = FALSE,
dates = c("2019-06-09", "2019-06-10")
)
library(tidyverse)
df
#> dates
#> 1 2019-06-09
#> 2 2019-06-10
df %>%
separate(dates, c("month", "year", "day"), sep = "-")
#> month year day
#> 1 2019 06 09
#> 2 2019 06 10
Created on 2019-06-10 by the reprex package (v0.3.0)