Go to "My Profile" at the top right corner of your screen and click on it. Once you do that, head all the way to the bottom of your profile page to find the signature box. That's where you can change it.
Shiny Rockersaid: and how do you change the Information that you can get by Examining the other player for example?Are you referring to the forum avatar? If so, you can only see what's available there: Number of posts, posts made by that user on that specific thread, their forum profile and RuneMetrics profile, if available to see.