Convert Northing, Easting to Latitude, Longitude.

I have two columns in an excel file, named Northing and Easting respectively. How can I convert them into longitude and latitude?

Work out what the original projected CRS is then convert it to a geographic CRS.

You could use st_transform() from sf.

For example, if it is already an sf object:

library(sf)

your_shp %>% 
  st_transform(4326)