Hi  nabilabolo,
Check this example. Now please take its reference and correct your code.
SQL
CREATE TABLE platform
(  
	platform_id INT IDENTITY PRIMARY KEY,
	platform_name VARCHAR(50),
	ph_no VARCHAR(50),
	baseType VARCHAR(50),
	isSelected BIT NOT NULL,
	EmailDate DATETIME
)
INSERT INTO platform VALUES('P1','1111','B1',1,GETDATE())
INSERT INTO platform VALUES('P2','2222','B2',0,GETDATE())
INSERT INTO platform VALUES('P3','3333','B3',1,GETDATE())
INSERT INTO platform VALUES('P4','4444','B4',1,GETDATE())
SELECT * FROM platform
Model
public class DetailModel
{
    public List<platform> platforms { get; set; }
}
Controller
public class HomeController : Controller
{
    PlatformEntities db = new PlatformEntities();
    public ActionResult Index()
    {
        DetailModel detailmodel = new DetailModel();
        detailmodel.platforms = db.platforms.Where(x => x.isSelected == true).ToList();
        return View(detailmodel);
    }
    [HttpPost]
    public ActionResult EmailedPlatform(DetailModel detailmodel)
    {
        var selectedPlatform = detailmodel.platforms.Where(x => x.isSelected == false).ToList();
        foreach (var item in selectedPlatform)
        {
            try
            {
                if (ModelState.IsValid)
                {
                    platform p = db.platforms.Where(x => x.platform_id == item.platform_id).FirstOrDefault();
                    p.isSelected = false;
                    p.EmailDate = null;
                    db.SaveChanges();
                }
                else
                {
                    ModelState.AddModelError("", "Update Fail");
                }
            }
            catch (DbEntityValidationException dbEx)
            {
                foreach (var validationErrors in dbEx.EntityValidationErrors)
                {
                    foreach (var validationError in validationErrors.ValidationErrors)
                    {
                        System.Diagnostics.Debug.WriteLine("Property: {0} Error: {1}", validationError.PropertyName, validationError.ErrorMessage);
                    }
                }
            }
        }
        return RedirectToAction("Index", "Home");
    }
}
View
<%@ Page Language="C#" Inherits="System.Web.Mvc.ViewPage<_174367_Update_CheckBox_MVC.Models.DetailModel>" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title>Index</title>
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.2/css/bootstrap.min.css" />
</head>
<body>
    <div>
        <%using (Html.BeginForm("EmailedPlatform", "Home", FormMethod.Post))
          { %>
        <div class="table" id="table_id">
            <table id="ItemList" class="table table-striped table-bordered" style="width: 100%">
                <thead>
                    <tr>
                        <th class="text-center">PH Number</th>
                        <th class="text-center">Platform Name</th>
                        <th class="text-center">Base Type</th>
                        <th class="text-center">Email Sent</th>
                    </tr>
                </thead>
                <tbody>
                    <%for (int i = 0; i < Model.platforms.Count; i++)
                      { %>
                    <tr>
                        <td class="text-center">
                            <%:Html.DisplayFor(m => m.platforms[i].ph_no)%>
                        </td>
                        <td class="text-center">
                            <%:Html.DisplayFor(m => m.platforms[i].platform_name)%>
                        </td>
                        <td class="text-center">
                            <%:Html.DisplayFor(m => m.platforms[i].baseType)%>
                        </td>
                        <td class="text-center">
                            <%:Html.HiddenFor(m => m.platforms[i].platform_id) %>
                            <%:Html.HiddenFor(m => m.platforms[i].ph_no)%>
                            <%:Html.HiddenFor(m => m.platforms[i].platform_name) %>
                            <%:Html.HiddenFor(m => m.platforms[i].baseType)%>
                            <%:Html.CheckBoxFor(m => m.platforms[i].isSelected)%>
                        </td>
                    </tr>
                    <%}%>
                </tbody>
            </table>
            <br />
            <input type="submit" value="Send Email" class="btn btn-success" />
        </div>
        <%}%>
    </div>
</body>
</html>
Screenshots

Database record after update
