Access 2013 update rows from multiple rows
You can't write a single query to do that in Access but a 'make table query' and a separate 'update query' combo will work. Change to suit your needs.

    select MyTable.RowID
        ,[t1].[MaxOfExtension] &amp; [t1].[MaxOfName] as NewValue
    into t2
    from MyTable
    inner join (
        select MyTable.MapID
            ,Max(MyTable.Extension) as MaxOfExtension
            ,Max(MyTable.name) as MaxOfName
        from MyTable
        group by MyTable.MapID
        ) as t1
        on MyTable.MapID = t1.MapID;

    update MyTable
    inner join t2
        on MyTable.RowID = t2.RowID
    set MyTable.MyNewField = [t2].[NewValue];