On the Settings popup, the Switch Accounts button allows you to toggle between up to two registered War Paradise accounts. You must first bind your current game to either your Google or Facebook account. If you're unsure how to find the Account popup, from your base view, tap your Commander's picture in the top left corner.
Once on the Profile page, tap the Settings button on the bottom navigation bar to open the Account popup. Tap Bind to begin registering your account.
Here you can center your Redeem Code. Keep in mind that codes are only good for a short period of time after they're available! The amount of time a Code stays valid will vary based on the value of the rewards tied to it; in any case where you've missed the time window, you'll see a 'Redeem Code Expired!' message when you try to redeem it. If the Redeem Code is valid, you'll see a 'Successful!' confirmation, and the rewards will be delivered to your account!