Navigate to YouTrack Settings -> Global Settings and click "Upgrade your subscription" button. You will be authorized in our shop and will be suggested to purchase a commercial plan.
Another option is to go to our website and select the required number of users there. After that, click "Buy new" or "Upgrade" buttons. You will be redirected to our store where you will be able to log in using your JetBrains Account credentials (and purchase a subscription after that) or to order a subscription without registering an account.
If it's "Buy new", fill in all required information - your email, subscription and credit card details and proceed to the checkout. Or, log in to your JBA account and proceed with the purchase. Please note that you will need a JBA account to manage your subscription or to delete your instance. You will be able to register a JBA account anytime later (please make sure you use the same email address).
If it's "Upgrade", enter your instance name, get a verification email to confirm your identity and proceed with the purchase.
If you face any problems, please don't hesitate to submit a support request, we'll be happy to help.