Structure( sorting by alphabet)

Question

Please, help to find my mistake. How to write toUpper function with char?

This is my code:
#include <iostream>
#include <fstream>
#include <vector>

using namespace std;

// create structure for train
struct train
{
int ID;
char* FromL0cation;
char* ToL0cation;
char* DepartureTime;
};

// custom comparator to do sorting
bool sort_by_to_location(train a, train b)
{
return a.ToL0cation < b.ToL0cation;
}

string t0UpperCase(string word)
{
// convert to uppercase
for (int i = 0; i < word.size(); i++)
{
if (word[i] >= 'a' && word[i] <= 'z')
{
word[i] = word[i] - 'a' + 'A';
}
}
return word;
}

int main()
{
// create a vector of planes
vector<train> arr;

// read input from command line
int n;
cin >> n;

// read n values
while (n--)
{
train p;

// read id
cin >> p.ID;

// read from
cin >> p.FromL0cation;

//read to
cin >> p.ToL0cation;

// read time
cin >> p.DepartureTime;

// convert from and to to uppercase
p.FromL0cation = t0UpperCase(p.FromL0cation);
p.ToL0cation = t0UpperCase(p.ToL0cation);

// add in vector
arr.push_back(p);
}

// sort using custom comparator
sort(arr.begin(), arr.end(), sort_by_to_location);

cout << endl
<< endl;
// show output
for (int i = 0; i < arr.size(); i++)
{
train x = arr[i];
cout << x.ID << " " << x.FromL0cation << " " << x.ToL0cation << " " << x.DepartureTime << endl;
}
}