How can I upgrade units?

[alley_cat]

Wow. It's benn years since I last visited Apolyton. I guess none here remembers me.
Well, anyway, here is my question:

I have lots of spearmans at the moment playing the role of guarding my cities. Now, I know you can somehow upgrade your exsisting units into more advance unit, if you have the appropriate tech. So I have the tech for pikeman, which I checked and saw it is the spearman upgrade. Plus, I built Leonardo's Workshop, joping it will some how help. Plus, I have all the needed recources connected to my city (in this case - Iron, which I know is connected since I see the Pikeman in the building list).
But still I found no way to upgrade my units, wether if they are in the city or outside.
How do I upgrade??

[Gramphos]

You can put the Spearmen in a city with Iron and Barracks and click on the lightning icon.