Note that you have to be the project owner to update a project.
Two options:
You want to change something on the project page itself. You just have to click "Main information" in your Toolbox and you’ll be redirected to the edition page of your project. You can make all the changes you want (add pictures, videos, sound files etc.) Then click "Publish" and the changes you made will appear on your project page.
You want to publish a News that will be sent to all your supporters and your followers. Click on the “News” tab on your project page. You’ll see a list of your latest updates, which are also visible to your donors. Click “Add a news,” and you’ll be redirected to the page where you compose your updates. You can create a news that includes text, photos, videos, or sound files of your choosing. Click “Publish” to publish your news. Your donors will all be notified by e-mail.