How to change a nominal variable to a numeric variable using if/else statements in R

I'm new to R. I imported a dataset and am trying to change the name of counties to a numeric value (FIPS code). I think I should use if/else statements but am unsure if this is the way to do it or how to do it. This is one column in the dataset:
County Yankton Fall River Tripp Bennett Beadle Brookings Brown

Thanks!