string command = "update table set id = '' where id =''";
fis.SubmitChanges();

------解决方案--------------------
private DataContext context = new DataContext();
   public void U_User(User user)
   {
       User userA = context.User.Single(c=>c.UserID == user.UserID);
       userA.UserID = user.UserID;
       userA.UserName = user.UserName;
       context.SubmitChanges();
   }



------解决方案--------------------
把aspnet_Users换为var试下
public string UpdateUsers(string empName, string empId, string PowerName, string ProvinceName)  
       {
           var users = fis.aspnet_Users.Single(p => p.UserName == empName);  
           users.PowerName = PowerName;  
           users.ProvinceName = ProvinceName;  
           users.empId = empId;  
           try  
           {  
               fis.SubmitChanges();  
           }  
           catch (Exception ex)  
           {  
               return "更新失败!";  
           }  
           return "更新成功!";  
       }

------解决方案--------------------
楼主，出错信息要发出来，这样谁知道是什么问题呀。

------解决方案--------------------
public static bool UpdateUsers(string empName, string empId, string PowerName, string ProvinceName)
       {
           try
           {
               var n=fis.aspnet_Users.Single(p => p.UserName == empName);

               n.PowerName = PowerName;
               n.ProvinceName = ProvinceName;
               n.empId = empId;
               db.SubmitChanges();
               return true;
           }
           catch
           {
               return false;
           }
       }
  
你这样写看对不对,
还有,你为什么以名字为查询条件,为什么不用ID,用名字,你确保是一条吗?若不是一条,就会出错
还有,我介意你,用实体传参public static bool UpdateUsers(实体名);