You sell the stock shares back to the Stock Market. It's right beside the option to buy more stocks. The Stock Market works by buying shares at a low price and then selling them when they get higher for a profit.
There is also a limit of 50,000 shares you can buy or sell each day. So make sure you're not trying to sell more than that number in one day or it will not work. When you're typing in the number, leave out any punctuation (50000 not 50,000)
Also, selling high would be somewhere in the 350s and over. That would give you more profit if you sold at around that range. Buying shares at one or two digits per share (i.e., 5, 19) would allow you to spend less in the long run. ^^