If you need to reset your Application Key, you can do so by signing into your account and navigating to the Buckets page. There, click to Show Account ID and Application Key.
Once you click to show the Account ID and Application Key, your Account ID will immediately be displayed. You will also have the option to create a new Application Key. Please keep note of the Application Key that you create so you can refer to it later.
Please Note: When you create a new Application Key, your old key will be invalidated. If you are using the B2 API in your scripts, you will need to make changes to account for the new Application Key. If you are using the Command Line tool, you'll need to make sure to re-authorize your account with the new Key as well.