要實現修改功能，首先需要獲取用戶輸入的數據，并根據用戶輸入的信息找到對應的數據進行修改。以下是一個簡單的示例代碼來實現修改功能：

#include <stdio.h>
#include <string.h>

struct Student {
    char name[50];
    int age;
};

int main() {
    struct Student students[10];
    int numStudents = 0;
    int choice, index;
    
    do {
        printf("\n1. Add student\n");
        printf("2. Modify student\n");
        printf("3. Exit\n");
        printf("Enter your choice: ");
        scanf("%d", &choice);
        
        switch(choice) {
            case 1:
                printf("\nEnter student name: ");
                scanf("%s", students[numStudents].name);
                printf("Enter student age: ");
                scanf("%d", &students[numStudents].age);
                numStudents++;
                break;
                
            case 2:
                printf("\nEnter index of student to modify (0-%d): ", numStudents-1);
                scanf("%d", &index);
                if (index >= 0 && index < numStudents) {
                    printf("Enter new name for student: ");
                    scanf("%s", students[index].name);
                    printf("Enter new age for student: ");
                    scanf("%d", &students[index].age);
                } else {
                    printf("Invalid index\n");
                }
                break;
                
            case 3:
                break;
                
            default:
                printf("Invalid choice\n");
        }
    } while (choice != 3);
    
    printf("\nList of students:\n");
    for (int i = 0; i < numStudents; i++) {
        printf("%d. %s, %d\n", i, students[i].name, students[i].age);
    }
    
    return 0;
}

這段代碼定義了一個學生結構體Student，可以添加學生信息和修改學生信息。用戶可以選擇添加學生信息、修改學生信息或退出程序。程序會一直運行直到用戶選擇退出。