Should be simple enough. To the right you probably see your handle when logged in. Undernearth that is a green text saying "Manage Profile". Click on that and it takes you to your list of posts and activity.
On this page, just about in the middle (across from your name on the left) at the top is another green text saying "Edit Profile". Click on this and you can enter your Bio there. Save it and you're done.
Hope that helps.
Gary