How to change date format to new format

I think this is silly questions.
How I would like to convert my current date format to new format.

Current format :
Factor: 2019_Jan, 2019_Feb, 2019_March, 2019_April

My desired format
Date: 2019/01, 2019/02, 2019/03, 2019/04

Thank you

It would be very helpful to have some actual sample data. See

or ?dput for some ways to do this.