Complaint.cs

public class Complaint 
{
     public int Id { get; set; }
     public string Reason { get; set; }
     
     public ComplaintFiler ComplaintFiler { get; set; }
}

HomeController.cs

public ActionResult Update(Complaint model)
{
    // Both the Reason and Id attributes should have been posted 
    // and will be present on the model
    
    // So now you can actually update your model (for example purposes
    // I'm doing this in the Controller, but you might have a more 
    // abstracted approach (e.g. a repository, etc.)

    // Grab your entity
    var original = _context.Complaints.Include(c => c.ComplaintFiler)
                                      .FirstOrDefault(c => c.COMP_ID == model.Id);
    if(original != null)
    {
          // We found your original complaint, so update it
          original.ComplaintReason = model.Reason;
          
          // If you wanted to update a related entity or child property that was included, you could do that here
          original.ComplaintFiler.Name = "Stupid complainer";
          
          // You would update other properties here as necessary
          
          // After all of your changes were made, you would save them
          SaveAll();
          
          return original;
    }
    
    // Your entity wasn't found, handle accordingly
    return HttpNotFound();
}

Index.cshtml

@model Complaint
<form action='@Url("Update","Home")'>
     @Html.TextBoxFor(m => m.Reason)
     @Html.HiddenFor(m => m.Id)
     <input type='submit' value='Update!' />
</form>