Select a product (Simple Start, Essentials, Advanced or Plus).
Choose Buy Now or select Try it free to subscribe.
Select Already have an account? Sign in.
Sign in with your previous or current user ID and password.
Follow the onscreen prompts to set up a new subscription under your existing login.
Once done, you can easily switch from one company file to another. Kindly refer to this article for the detailed steps: How do I switch companies?.
If you wish to learn more about the things you need to do with your new company, you can visit this article link for the detailed information: Learn QuickBooks basics and get set up for success. It contains topics and subtopics that are very helpful in setting up your company.
Please let me know if you have other concerns. I'm just around to help.