Easy--go up to that tool bar at the top of your TT screen and click the green circle with the arrow in it that is right above the word "Upgrade." You will just be charged the difference between the price for Premier and Self-Employed.
Assuming that you purchased the TurboTax software to be installed on a personal computer.
All the TurboTax desktop CD/Download editions contain the same forms, schedules and worksheets. The difference between the editions is the amount of help received in the various section of the program.
You can complete a Schedule C for self-employment income and expenses using the TurboTax Premier desktop edition. The program will recommend that you upgrade to Home & Business but that is only a recommendation and Not a requirement.
If you do want to upgrade to Home & Business then just click on the Upgrade TurboTax link at the bottom right of the program screen. You will be asked to purchase the upgrade which will be the difference in price between the Premier edition and the Home & Business edition.