Click the "My Profile" tab at the top of this or most other BYC pages. Your Profile page will show up, in the top right corner of your profile are two (Gray) Bars "Edit Community Profile" and "Edit Account Details" side by side. Click the "Edit community Profile" bar. Your Community profile page will show up: Scroll about 3/4 of the way down the page and there is a place to add or change your signature. After the editing click the (Blue) SAVE button and you are done.
Hope this helps.