Each SharePoint list by default has these fields 'Created By' and 'Modified By'. The type of these fields  is "Person or Group". Whenever a new list item is added, the value of the 'Created By' column is automatically set to the LoginName of current user. And whenever a list item is modified, the value of the 'Modified By' column is automatically set to the LoginName of current user.

So if one need to modify the values of these fields, I did not find any work flows or any UI screen in the SharePoint yet.

Now the question is "why the hell one need to modify the values of these fields?". The answer can be found in the "Advanced Settings" section of a SharePoint list. One can see, there is a 'Edit own items' permission which means a user can edit all list items which he/she has created, that is very nice and useful feature. But the problem is that it is linked with the 'Created By' field. So if i will create a list item then my login name will be stored in the "Created By" field and only I can modify this list item. But if there is an requirement to modify this list item by some other user then how I can achieve that. To achieve this, one need to modify the value of "Created By" field and set it to the login name of that user. But How?

I was just stuck with this problem, but finally found a solution. The values of 'Created By' and 'Modified By' fields can be modified programmatically. Here is a code snippet to achieve this:

using (SPSite site = new SPSite("SharepointSiteURL"))

{ 

  using (SPWeb web = site.OpenWeb())

  {

    web.AllowUnsafeUpdates = true;

    SPList list = web.Lists["ListName"];

    foreach (SPListItem item in list.Items)

    {

      SPUser user = web.EnsureUser("User'sLoginName");

      //In the "Created By" and " Modified By" fields values are stoted in this format "ID;#LoginName", so create the same format

      string
userValue = user.ID + ";#" + user.Name;

      item["Author"] = userValue ; //InternalName of "Created By" field is "Author"

      item["Editor"]=userValue //InternalName of "Modified By" field is "Editor"

      item.Update(); 

    }

    list.Update();

    web.Update();

  }

}

Have fun and keep coding.............