#include<bits/stdc++.h>
using namespace std;

int main()
{
	int t;
	cin>>t;
	while(t--)
	{
	int n;
	cin>>n;
	string s;
	cin>>s;
	int a[26]={0};
	for(int i=0;i<n;i++)
	{
		a[s[i]-'a']++;
	}
	pair<pair<int,char>,int>low,high;
	low=high={{a[s[0]-'a'],s[0]},0};
	for(int i=1;i<n;i++)
	{
		low=min(low,{{a[s[i]-'a'],s[i]},i});
		high=max(high,{{a[s[i]-'a'],s[i]},i});
	}
	s[low.second]=s[high.second];
	cout<<s<<"\n";
	}
}