How to improve creatures in Heroes 3.58 (WOG)?

Please help, I can't figure out how to upgrade creatures in Heroes 3.8, for example, how to turn a centaur into an archer?

First of all, it's not 3.8, but 3.58 :)
Secondly, Sylvan centaurs (as I understand, that's what you're talking about) are upgraded as follows:
You need to have Great Elves and Centaur Captains in your army. In the hero's window, hold down Ctrl and click on one of them. You will be offered to upgrade them. :)