1- Click on the Profile link at the top of the page
2-You will see a new window popping up "Registration Information". You will see a list of link: Information | Preferences | Signature | Avatar | Friends and foes | Watched topics. Click on the link you want to update!
3-To add an avatar or signature, you should upload a picture and then put the link on the relevant box
4- When you click on 'save', your new profile is updated!
Hope this helps, please do not hesitate to ask if you need help