Yup its the upgrade symbol

If you have a single ship selected(not stacked) there should be an upgrade button left of the ship thumbnail in the UI. If they are stacked use the details button, then you'll see the upgrade option in the details screen. If they're fleeted you have to go to the manage button that replaces the detail button. In the popup box select a ship and choose details toward the lower right. Or alernately you can just hit D when a ship is seleted to get the details screen.
Note: You can only upgrade to other models with same size hull. No freighter to battleship conversion   

BTW If there is an easier way to upgrade fleeted ships or be able to upgrade entire fleets at once, I'd love to hear it.

This upgrades the design to be built, but you will still have to manually upgrade ships already in the field.btw: You can upgrade an entire class at a time using the little tick box toward the upper left of the ship upgrade screen. Just make sure you have the money for it. It can be expensive if you have a lot of ships of that class.

Thats an interesting idea. I'd like to find out if its possible to upgrade a constuctor to a colony ship and whether it creates "free" pop?

You will end up with a colony ship with 1m on board. It's good if you discover a planet that you can't get a colony ship to quick enough, but you'll have to bus some more pop to it to get it to grow at a decent rate.



Actually quick building a blank hull and upgrading it can save some cash over quick building the ship outright. How much depends on the specific ship and circumstance.