Up to three times per day, the wearer can change the energy damage of a fire spell she casts to cold, a cold spell to fire, an acid spell to electricity, or an electricity spell to acid.
It seems to me it is just part of the spell beeing cast and requires no action. My GM ruled in the last session it requires a standard action and affects the next appropriate spell.
Activating a magic item is a standard action unless the item description indicates otherwise.
An interesting predicament. Strictly speaking, you're not activating the item, though; it's conferring onto you the ability to make the changes, comparable and perhaps similar to a Metamagic Rod of Elemental Spell.
So it's active simply by being worn, you don't need to turn it on or off.