You can access and modify your War Decks by switching to the Clan Wars tab and clicking on the Clan button twice at the bottom of your screen in Clash Royale. From here, click on any active events ...

What are War Decks in Clash Royale War Decks, also known as Duel Decks, are four decks with 32 unique card combinations that you can use to participate in the weekly Clan battles.