Express system can upgrade to Practice Pro, this is useful if you want features that Express doesn't have, these include Document management, Client portal, and Customized reporting to name a few.
How to upgrade
Hover on top of you company name and click on Subscription Settings, there be an option to click on Learn more & upgrade.
You would get a preview of your new bill, once you're happy to proceed then tick the disclaimers and click Confirm upgrade.
You can only upgrade from Express to Practice Pro and not Practice Pro + Accounting