package main

import (
	"fmt"
)

func main() {
	var t int
	fmt.Scan(&t)
	for i := 0; i < t; i++ {
		var n int
		var s string
		fmt.Scan(&n, &s)
		counts := make(map[rune]int)
		for _, c := range s {
			counts[c]++
		}
		maxC, minC := -1, 100
		var maxChar, minChar rune
		for c, count := range counts {
			if count > maxC {
				maxC = count
				maxChar = c
			}
			if count < minC {
				minC = count
				minChar = c
			}
		}
		res := []rune(s)
		for j, c := range res {
			if c == minChar {
				res[j] = maxChar
				break
			}
		}
		fmt.Println(string(res))
	}
}