Convert year and day of year to month

I have files that are named according to year and day of year. How do I convert day of year to month?

Basic example:

library(tidyverse)
library(lubridate)
(mydates <- 
  tibble(year = c(2010, 2020),
          dayofyear = c(001, 365),
         yearday = c(2010001, 2020365)))


mydates %>% mutate(dayofyear = yday(dayofyear))
mydates %>% mutate(yearday = yday(yearday))