If you want to go all base R, here's a suggestion. I assume that the original Country column is called "country":
top_countries = rdf[!is.na(rdf$country),]
top_countries = top_countries[order(top_countries$geo_country, decreasing=T),][1:6,]
colnames(top_countries) <- c("Country", "Frequency")
Of course, this can be done more elegantly with tinyVerse/dplyr
rdf %>% filter(!is.na(country)) %>% arrange(desc(geo_country)) %>% rename(Country=country, Frequency=geo_country) %>% top_n(6)