Editing existing content on your WordPress site is very easy. The first thing you need to do is login. View the article 'Logging in to the backend/admin area of WordPress' for instructions on how to do this.
Once logged in, if you click the site's title in the top left corner, you will be taken to the front end of the website. You'll see a black bar at the top.
To edit most pages, you can simply visit them, and then click the edit link at the top.
Sometimes this will say 'Edit Post', or perhaps 'Edit Article'. Either way, it takes you to the edit page for that particular web page you're viewing.
The edit page will look similar to this:
Here you can edit text. You have a lot of the editing features you may be familiar with from programs like MS Word (make text bold, italic, add lists, etc.).
When you're done editing, hit the Update button. (it may say publish if creating a brand new page). Clicking 'View Page / Post / Article' at the top will take you back to the front end so you can view your page.