Here is a solution using lubridate
.
library(lubridate)
#Set the desired format (mm-yyyy) as my_stamp
my_stamp<-stamp( "02-2019",
orders = "my")
#A df with a column full of dates
df <- data.frame(dates = c("30/04/2020","29/03/2020","28/02/2020"))
#Change the column from string to date format
df$dates<-dmy(df$dates)
#Apply the format you desire to the dates (i.e., only month and year)
df$dates<-my_stamp(df$dates)
# dates
#1 04-2020
#2 03-2020
#3 02-2020