Sorry for the late reply
To change your signature I believe you go to Settings and the section that says "Comment Signature" is where you go to change your signature.
I hope this helps
It is in the header, the green thing/ribbon on the top of website. Its with: deviantart, Searchbar, Deviant (Your Username), Messages, Submit, Shop, Upgrade, Friends, and Favourites all in it in that order.
Next to Deviant (Your Username) there is an arrow or upsidown triangle next to it. Hover your mouse over it or click on it.
It will a little menu with: Profile, Upgrade Now, Settings, and Logout. Just click on the "Settings".
That should be how you can get to "Settings"
Sorry for the late reply, I have been busy