Convert lowercase letters to uppercase

#include <iostream>
#include <string>
using namespace std;
// Convert lowercase letters to uppercase.
int main()
{
    string word; // This is the input.

    cout << "Enter a word:" << endl;
    cin >> word; // Get the word from the user.

     // Check each letter.
    for (size_t i = 0; i < word.size(); ++i)
    {
        // If the letter is lowercase,
        if (word[i] >= 97 && word[i] <= 122)
        {
            // Change it to uppercase.
            word[i] = word[i] - 32;
        }
    }

    // Display the converted word.
    cout << word << endl;

    return 0;
}