Editing a page
Editing a page is very similar to how pages are added.
First, make sure you're logged in - only those with the appropriate permissions can edit content.
Next, go to the page you want to edit. On this page you should see a series of tabs. The number and type of tabs available varies according to modules installed. But as long as you have the correct permissions, you should at least see two tabs - View and Edit.
Once you click on the Edit link, you'll get a page that looks just like the content creation page - and works the exact same way - except your content is already there.
You can also go to the listing of content and edit items from there as well.
Go to Content on the admin menu.
This will give you a listing of all the content on your site. Click on the edit link to the right and it will take you to the edit page for that piece of content.