数据库的字段设置主键字段没有？GridView控件自带的更新功能要求数据库要有主键字段来做一个某条数据的唯一标识的！
如果你设置了主键字段如将编号id设置为主键，程序中的id值如果你这样设置：
//获取文本框的值
int id = int.Parse(GridView1.Rows[e.RowIndex].Cells[0].Text.Trim());
那么你把GridView控件的DataKeyNames属性设置为id，即主键字段！
数据更新它得要找你更新的数据的主键，找不到就不会更新成功！
还有你对页面时行初始化即对Gridview控件中数据绑定时要在Page_Load事件中加上If(!IsPostBack){...}来首先判断下页面是否首次加载！

示例：
<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False" AllowPaging="True" 
                                          OnPageIndexChanging="GridView1_PageIndexChanging" PageSize="12" 
                                          OnRowDataBound="GridView1_RowDataBound"　BackColor="White" Font-Size="13px" 
                                          Width="541px" [strong]DataKeyNames="ID"[/strong]
                        OnRowCancelingEdit="GridView1_RowCancelingEdit" OnRowDeleting="GridView1_RowDeleting" 
                                          OnRowEditing="GridView1_RowEditing" OnRowUpdating="GridView1_RowUpdating" 
                                          SkinID="gvSkin" >
                        <Columns>
                            <asp:TemplateField HeaderText="编号">
                                <ItemTemplate>
                                    <asp:Label ID="Label1" runat="server"><%# Eval("ID") %></asp:Label>
                                </ItemTemplate>
                            </asp:TemplateField>
                            <asp:TemplateField HeaderText="考试科目">
                                <EditItemTemplate>
                                    <asp:TextBox ID="txtName" runat="server" Text='<%# Eval("Name") %>' Width="80"></asp:TextBox>
                                </EditItemTemplate>
                                <ItemTemplate>
                                    <asp:Label ID="Label2" runat="server"><%# Eval("Name") %></asp:Label>
                                </ItemTemplate>
                            </asp:TemplateField>                                                     
                            <asp:CommandField HeaderText="编辑" ShowEditButton="True" />
                            <asp:TemplateField HeaderText="删除" ShowHeader="False">
                                <ItemTemplate>
                                    <asp:LinkButton ID="LinkButton1" runat="server" CausesValidation="False" CommandName="Delete" OnClientClick="return confirm('确认要删除吗？');" 
                                        Text="删除"></asp:LinkButton>
                                </ItemTemplate>
                            </asp:TemplateField>
                    </Columns>
                </asp:GridView>
后台部分代码：
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            InitData();  //初始化考试科目          
        }
    }
    //初始化考试科目
    protected void InitData()
    {
        Coursemr course = new Coursemr();       
        DataSet ds = course.QueryCourse();  
        GridView1.DataSource = ds;          //为GridView控件设置数据源        
        GridView1.DataBind();               //绑定数据
    }
    //GridView控件RowUpdating事件
    protected void GridView1_RowUpdating(object sender, GridViewUpdateEventArgs e)
    {
        int ID = int.Parse(GridView1.DataKeys[e.RowIndex].Values[0].ToString()); //取出要更新记录的主键值
        Coursemr course = new Coursemr();           //创建Coursemr对象
        course.Name = ((TextBox)GridView1.Rows[e.RowIndex].FindControl("txtName")).Text;
        if (course.UpdateByStr(ID))//使用Usersmr类UpdateByProc方法修改用户信息
        {
            Response.Write("<script language=javascript>alert('修改成功!')</script>");
        }
        else
        {
            Response.Write("<script language=javascript>alert('修改失败!')</script>");
        }
        GridView1.EditIndex = -1;
        InitData();
    }

int id = int.Parse(GridView1.Rows[e.RowIndex].Cells[0].Text.Trim());

把上句改成

string id=GridView1.DataKeys[e.RowsIndex].Value.Tostring();

试试