Grant Option allows you to either change the stats of an equipment to another option, or add it to another empty slot.
You can tap on the [Equipment Modification], [Grant Option] menu from the main menu (≡) to view which equipment is eligible, and also to see the amount of Option Stones you have.
You can tap on an available option slot or tap on the existing extra stats of the equipment you would like to use the Grant Option on, and then place the Option Stone and material to grant the option.