One of the lateral incisor is slightly inside. What are the possible techniques to align the teeth to have a nice smile? (photo)

Yes, I agree that your issues are minor.  If, however, it's important to you, then go ahead and do something to make it better.Actually, this is pretty easy to fix with an old fashioned Hawley retainer (named after a Dr. Hawley).  Chances are good that when you had your braces the first time, you were given a Hawley retainer for the upper teeth. Typically, they have plastic in the roof of the mouth and a wire on the outside of the front teeth.  What we do in our office is design a modified Hawley.  It looks pretty much the same except there are a few extra wires in various places.  For example, we would put a wire behind the lateral incisor that will rotate it back into the proper alignment. Usually, for our adult patients, we ask them to wear the retainer at home in the evening and to sleep at night.  The more you wear it, the faster the tooth moves.  After it's in the right place, then you either continue night time wear to hold it there or have a bonded permanent retainer placed (a wire on the back side of the front teeth). Keep in mind that if you just stop wearing the retainer, the tooth will probably move back to the way it is now.Good luck!