First of all, you need to be logged in.....
You should not go to the "About me". This will only display info, not give you the opportunity to edit the info.
Click on the "Clipboard"-icon in the upper right-hand corner to get to the User Control Panel.
Along the left-hand side you will find "Edit Signature".
This will give you the opportunity to edit and preview your signature before saving it.
Hope this points you in the right direction?