Change Password in ASP.NET MVC identity, without old password?

Yes, you can do that, you can change password in MVC using ASP.NET identity without needing old password of user, so here the code for Code it.

[HttpPost]
        [ValidateAntiForgeryToken]
        public async System.Threading.Tasks.Task<ActionResult> UpdatePassword(FormCollection formCollection)
        {
           
                //get user by it's unique ID
                ApplicationUser user = await UserManager.FindByIdAsync(Convert.ToInt32( formCollection["Id"]));
               
                // Use passwordHash to add new password
                user.PasswordHash = UserManager.PasswordHasher.HashPassword(formCollection["Password"]);
                //update user password
                var result = await UserManager.UpdateAsync(user);
                 
                if (result.Succeeded)
                {
                   // do something
               }
            }
            return RedirectToAction("SomeAction");
        }

Where, FormCollection is getting User Id and New password from user.

Your View.cshtml can be like this

 @using (Html.BeginForm("UpdatePassword", "ControllerName", FormMethod.Post, new {  @class = "validatedForm" }))
        {
            @Html.AntiForgeryToken()
              
            <input type="hidden" id="Id" name="Id" value="@ViewBag.Id" />

            <div class="form-group">
                @Html.Label("Email")
                <input type="text" value="UserEmail"  class="form-control"  id="Email" name="Email" disabled  />


            </div>

            <div class="form-group">
                @Html.Label("Password")
                @Html.TextBox("Password", "", new { @class = "form-control", type = "password"})

            </div>

            <div class="form-group">
                @Html.Label("Confirm Password")
                @Html.TextBox("ConfirmPassword", "", new { @class = "form-control", type = "password" })


            </div>



            <div class="form-group">

                <input type="submit" value="Update" class="btn btn-primary" id="SubmitBtn" />

            </div>
        }

OR

If you don't want to use PasswordHasher without any validation, try this

//Get User By Id
var user = await UserManager.FindByIdAsync(id);

//Generate Token
var token = await UserManager.GeneratePasswordResetTokenAsync(user);

//Set new Password
var result = await UserManager.ResetPasswordAsync(user, token, "YouN3wP@ssw0rd");

You can also check Forgot password functionality in MVC.