Within the app there should be a menu (either a "hamburger" or 3-bar menu on the left or a 3-dot menu on the top right) on the menu should be a way to sign out (if the developer did it right), sign out then create your new account. Be aware that any lists you may have saved were likely tied to the account so they'll be gone.
Another option is to find it in the app manager (settings > apps or apps and notifications depending on the device and OS) and clear the cache and data to effectively reset the app to 0.
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