Genesect does not have Form changes. What it can change is the type of it's signature move, Techno Blast. If you give it one of it's Drives, Techno Blast will change in type accordingly.
You get the Drives from Scientist Dudley at P2 Laboratory on Route 17. He will battle You must have Genesect in your party. He will gives you 2 drives (depending on which version you are playing) after being defeated.
His Pokemon are Klink & Klang, both Level 34.
No Drive = Normal
If you are playing Black or Black 2:
(Burn Drive = Fire)
(Shock Drive = Electric)
If you are playing White or White 2
(Douse Drive = Water)
(Chill Drive = Ice)