Simply go to Content>Article Manager and click on the link in the Title column (3rd column)

This will open the page for editing. While you're working on the page, you can click 'Apply' at the top left. This saves the page without closing it, so that you can continue working on it. Once you've finished, just click 'Save'.

If you ever click on your back button for your browser, this doesn't close the page correctly and the article will be locked in the Article Manager. To remove the lock on an article, simply go to Tools>Global Checkin.