[AllowAnonymous]
public async Task<ViewResult> ResetAllPasswords()
{
    // Get a list of all Users
    List<ApplicationUser> allUsers = await db.Users.ToListAsync();
    // NOTE: make sure this password complies with the password requirements set up in Identity.Config
    string newPassword = "YourNewPassword!";
    int passwordChangeSuccess = 0;
    int countUsers = 0;
    // Loop through the list of Users
    foreach (var user in allUsers)
    {
        // Get the User
        ApplicationUser thisUser = await UserManager.FindByNameAsync(user.UserName);
        // Generate a password reset token
        string token = await UserManager.GeneratePasswordResetTokenAsync(thisUser.Id);
        // Change the password, using the reset token
        IdentityResult result = await UserManager.ResetPasswordAsync(thisUser.Id, token, newPassword);

        // Record results (extend to taste)
        if (result.Succeeded)
        {
            passwordChangeSuccess++;
        }
        countUsers++;
    }

    ViewBag.CountUsers = countUsers;
    ViewBag.PasswordSuccess = passwordChangeSuccess;

    return View();
}