There is currently no way to change the characters in battle until you have this item, so just keep playing without this feature if you haven't already gotten to Dahngrest.
Anyway, you can synthesize the Moon Selector for the following items:
- 1,000 Gald.
- 3 x Insect horns.These items drop from most insect type enemies, so focus on finding and killing these if you don't already have the three you need.
- 1x Red Lens. This item drop from killing "Red Eyes" enemies.
When you have these items (and enough Gald), go to the Fortune's Market item shop and synthesize the Moon Selector.
Changing characters in battleWith the Moon Selectors in your inventory, you are able to change your characters in battle, and you won't need to equip or active the Moon Selector. It just works when you have it.
When you are in a battle, open the main menu (X on Switch, Y on Xbox, or Triangle on PS4) and click on the start button. This will cycle your characters one spot towards the left, so you can repeatedly click the start button to change it to be like you want it to.