Hi  nabilabolo,
Check this example. Now please take its reference and correct your code.
SQL
CREATE TABLE WorkDetails
(
    Id INT IDENTITY PRIMARY KEY,
    WorkWeek INT,
    DDPM INT
)
Model
public class WorkModel
{
    [Required]
    public int WorkWeek { get; set; }
    [Required]
    public int DPPM { get; set; }
}
Controller
public class HomeController : Controller
{
    // GET: /Home/
    public ActionResult Index()
    {
        TestEntities entities = new TestEntities();
        TempData["Details"] = entities.WorkDetails.ToList();
        return View(new WorkModel());
    }
    [HttpPost]
    public ActionResult Index(WorkModel work)
    {
        if (ModelState.IsValid)
        {
            TestEntities entities = new TestEntities();
            WorkDetail workDetail = entities.WorkDetails.Where(x => x.WorkWeek == work.WorkWeek).FirstOrDefault();
            if (workDetail == null)
            {
                // Insert new record.
                workDetail = new WorkDetail();
                workDetail.WorkWeek = work.WorkWeek;
                workDetail.DDPM = work.DPPM;
                entities.WorkDetails.AddObject(workDetail);
                entities.SaveChanges();
            }
            else
            {
                // Update record based on WorkWeek.
                workDetail.DDPM = workDetail.DDPM + work.DPPM;
                entities.SaveChanges();
            }
            return RedirectToAction("Index");
        }
        return View(work);
    }
}
View
<%@ Page Language="C#" Inherits="System.Web.Mvc.ViewPage<Insert_Update_MVC.Models.WorkModel>" %>
<%@ Import Namespace="Insert_Update_MVC" %>
<!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>
</head>
<body>
    <% using (Html.BeginForm()) {%>
    <table>
        <tr>
            <td>
                <%: Html.LabelFor(model => model.WorkWeek) %>
            </td>
            <td>
                <%: Html.TextBoxFor(model => model.WorkWeek) %>
                <%: Html.ValidationMessageFor(model => model.WorkWeek) %>
            </td>
        </tr>
        <tr>
            <td>
                <%: Html.LabelFor(model => model.DPPM) %>
            </td>
            <td>
                <%: Html.TextBoxFor(model => model.DPPM) %>
                <%: Html.ValidationMessageFor(model => model.DPPM) %>
            </td>
        </tr>
        <tr>
            <td colspan="2" align="center">
                <input type="submit" value="Save" />
            </td>
        </tr>
    </table>
    <% } %>
    <hr />
    <%if (TempData["Details"] != null) { %>
    <table>
        <tr>
            <th>No.</th>
            <th>Work Week</th>
            <th>DPPM</th>
        </tr>
        <%foreach (WorkDetail wd in (List<WorkDetail>)TempData["Details"])
          {%>
        <tr>
            <td><%= wd.Id%></td>
            <td><%= wd.WorkWeek%></td>
            <td><%= wd.DDPM%></td>
        </tr>
        <% } %>
    </table>
    <%} %>
</body>
</html>
Screenshot
