How do you change grand company?

I am maxed rank, I purchased my miniature Aetheryte for when I have a house and I also got 10000 serpent seals (is what I heard you need to transfer)

But I talked to every npc and none of them tell me how to switch?

Not sure if anything changed, to transfer you need to pay 50k gil.

Go visit the Personnel Officer of the GC you wish to switch and there is an option that says "Apply for Transfer"

First time is free, 50k every time after that.
Just speak to one of the guys in the company you want to switch to.