图看不准，给代码看看。

报错信息很明显，score_modify重复定义

void score_modify(struct STUDENT *Wstu, int *num)
{
    int m3, i3;
    long long op_3;
    char op3_3[20];
    float op2_3;
    printf(" 1.按学号修改\n 2.按姓名修改\n 0.退出\n");
    do
    {
        printf("请输入你的选择:");
        scanf("%d", &m3);
        if(m3 == 1)
        {
            printf("请输入要修改的学生的学号:");
            scanf("%l64d", &op_3);
            for(i3 = 0; Wstu[i3].stu_ID != op_3 && i3 < *num; i3++);
        }
        else if(m3 == 2)
        {
            printf("请输入要修改的学生的姓名:");
            scanf("%s", op3_3);
            for(i3 = 0; strcmp(Wstu[i3].stu_name, op3_3) != 0 && i3 < *num; i3++);
        }
        printf("修改前:%l64d %s %.1f\n",Wstu[i3].stu_ID,Wstu[i3].stu_name,Wstu[i3].score);
        printf("输入你想修改成绩为:");
        scanf("%f",&op2_3);
        Wstu[i3].score = op2_3;
        printf("\n修改后:%l64d %s %.1f\n\n",Wstu[i3].stu_ID,Wstu[i3].stu_name,Wstu[i3].score);
    }while(m3);
    printf("修改成功\n");
}

/**************************************/
#ifndef __CHOSE_H__INCLUDE
#define __CHOSE_H__INCLUDE

#define N 100
struct STUDENT
{
    long long stu_ID;
    char stu_name[20];
    float score;
}student[N];

void meun();//菜单
void grade_add(struct STUDENT *Wstu, int *num);//学生成绩新增
void score_inquiry(struct STUDENT *Wstu, int *num);//学生成绩查询
void score_modify(struct STUDENT *Wstu, int *num);//学生成绩修改
void grade_delete(struct STUDENT *Wstu, int *num);//学生成绩删除子函数
void grade_all(struct STUDENT *Wstu, int *num);//学生成绩全览
void grade_statisticsc(struct STUDENT *Wstu, int *num);//班级成绩统计
void yesorno();//退出

#endif

看看其他地方还有无这句：
void score_modify(struct STUDENT *Wstu, int *num);

没有

先调试一下框架，如：
/* chose.h */
#ifndef __CHOSE_H__INCLUDE
#define __CHOSE_H__INCLUDE

#define N 100

struct STUDENT
{
    long long stu_ID;
    char stu_name[20];
    float score;
}student[N];

void score_modify(struct STUDENT *Wstu, int *num);//学生成绩修改

#endif


/* chose.c*/
#include <stdio.h>
#include "chose.h"
main()
{
    score_modify(NULL, NULL);
}

void score_modify(struct STUDENT *Wstu, int *num)
{
    puts("score_modify");
}

嗯，谢谢