If you have used the PXT editor before, it will remeber previous code, clicking on the 'New Project' button will allow you to make new code.
When the new project loads you will see two blocks on the page, an 'on start' and 'forever' block, delete these block by draging them over the tabs to the left
Next we need to change the name of the script, click in the 'Untitled' box located next to the download button at the bottom of the page. This will allow you to change the name to something meaningful i.e. Flashing LEDS