How to update Modified By field in a SharePoint list item programatically

While adding/updating a list item using SPSecurity.RunWithElevatedPrivileges with administrator privileges, Created By / Modified By fields are set to System Account as the piece of code is executed with Application Pool account context.

But in an ideal case Created By / Modified By fields should have the original user who actually did the modification. In order to achieve this Created By / Modified By fields needs to be updated at the time of inserting / updating list item.

The catchy point here is to get the Current User out of the SPSecurity.RunWithElevatedPrivileges scope because within the scope it is always System Account as mentioned.

The below code snippet works as as explained above:

////Get the current user out of the SPSecurity.RunWithElevatedPrivileges scope
SPUser currentUser = SPContext.Current.Web.CurrentUser;

SPSecurity.RunWithElevatedPrivileges(delegate()
{
    using (SPSite newSite = new SPSite("http://siteurl"))
    {
        using (SPWeb newWeb = newSite.OpenWeb())
        {
            SPList list = newWeb.Lists.TryGetList("List Title");
            if (null != list)
            {
                bool isNewItem = true; //Adding or Updating item
                SPListItem item = list.GetItemById(1);
                if (isNewItem)
                {
                    item["Author"] = currentUser;
                }
                item["Editor"] = currentUser;

                item.Update();
            }
        }
    }
});

Reference:
How to update Modified By field programatically
How to update Created By field programatically
SharePoint List Item update Modified By field programatically
SharePoint List Item update Created By field programatically