The "current tank info" part of your sig is separate from your "signature."
Up toward the top left is your "User cp" - go in there
On the left side is an "edit your details" link, inside a "your profile" box. It's a few spots above the one where you edit your sig
Way down after the "are you a bot?" stuff, where you can put info about where you live etc. is a box for "current tank info"
Whatever you put there shows up under your sig proper that you have been editing. If you change you current tank info it will put the new info under your sig.
It's nowhere near as hard as I made it sound once you find the box lol. I guess it's so you can keep your sig and change your tanks, or vice versa?