var Db = new ApplicationDbContext();
var userDetail = Db.Users.FirstOrDefault(u => u.UserName.Equals(Username));

String userId = userDetail.Id;
String newPassword = "pa$$word";
ApplicationUser cUser = UserManager.FindById(userId);
String hashedNewPassword = UserManager.PasswordHasher.HashPassword(newPassword);
UserStore store = new UserStore();
cUser.PasswordHash = hashedNewPassword;
UserManager.UpdateAsync(cUser);
Db.SaveChanges();