Summarize:

If you're not automatically logged in, log in with your Facebook account. You will have to enter your email or phone number and your password. Your name and a thumbnail of your profile picture will be at the top of the navigation menu towards the upper-left corner of your Home page. Clicking on this button will take you to your Profile. A pencil icon will appear next to it. This is the Edit button. You will begin editing your Intro. Think about how you want to introduce your profile to visitors, and type in an Intro statement for yourself. This button will be right below the Intro text field, and it will save the changes to your Intro.
Open Facebook.com in a web browser. Click on your name on the left navigation menu. Hover over your current Intro. Click the pencil icon. Edit your Intro. Click Save.