新人学C++,不知道哪出错了，请大侠帮忙

#include<iostream>
using namespace std;
class Student
{
    public:
        Student(int n,float s):num(n),score(s){}
        void display();
        void change(int m,float n);
    private:
        int num;
        float score;
};
void Student::display()
{
    cout<<num<<" "<<score<<endl;
}
void Student::change(int m,float n)
{
    num=m;
    score=n;
}
int main()
{
    Student stu1(1,90);
    stu1.display();
    Stu1.change(2,80);
    Stu1.display();
    return 0;
}
编译结果：
D:\新建文件夹\9\4.cpp(26) : error C2065: 'Stu1' : undeclared identifier
D:\新建文件夹\9\4.cpp(26) : error C2228: left of '.change' must have class/struct/union type
D:\新建文件夹\9\4.cpp(27) : error C2228: left of '.display' must have class/struct/union type