Upgrading Units

[HamTard]

I could've swore I did it in the first game I played, but in the second game I couldn't find the button. What are the conditions required to be able to upgrade a unit and where does the button appear?

 

[Grunthex]

Unit has to be in your territory, there has to be a valid upgrade (Warriors don't get to be spearmen no matter how hard you beg, they have to wait for swords). It appears in your unit menu to the left, right in the main part of the menu, not even the hidden part.