If you are using TurboTax Online then the version will update automatically when you reach a section in your return that the Deluxe version cannot handle, such as a Schedule C with expenses to claim.
If you are using a CD/downloaded version, there is no way to upgrade without purchasing the higher version. However, all forms are included in the Deluxe version. The guidance is more advanced in the higher version of the program, but you can still access anything you need in Deluxe.