Once you select how you want to prepare your return, you cannot change it. If you just selected to create the return for yourself, you cannot go back and change it (you'll be able to click the Back button but you won't be able to change how you chose to prepare your return on that screen). If you now want to file jointly, go to Tools > Return Manager > Create new return. Then when you're asked how you want to prepare your return, select "My spouse/partner and me".
