You can buy stocks in Accounts > Stocks & Shares ISA > Add more funds or stocks > Stocks. Once you’ve selected a stock, you can tap ‘Invest weekly’ if you’d prefer to set up a weekly stock buy.
Once you’ve invested in stocks, you can request a sell at any time in the Accounts tab by tapping the stock you’d like to sell. If you have a weekly stock buy set up, this can be changed in Settings > Weekly deposit.