I used a Peta water in my Galactic Lain and he beacme a Experiment, how i chance back?

You're unfortunately going to have to wait until the sixth or seventh of next month to get your pet blessed at the Coda Caves to change it back to Galactic. I'm not aware of any other way to get a Galactic pet.

I pearphs can chance he back to a common Lain?

Find a cheap lain elixir, arid is cheap (around 16k or so)