driver door, 2000 vr6

my driver side door wont unlock or open from the outside. i have to open the back door and reach to the front to open it. as soon as i pull the lever from the inside the door unlocks and opens. but as soon as i shut the door it locks on its own as soon as i close it. can i just get a new door lock mechanism and replace it myself or do i have to take it in and have it done.