how to upgrade unit ?

[davide]

I bought this game yesterday and ...
... I did little hours tonight: a typical old style Turn-Based night!!!
... an other turn ...an other turn ...and so on

I do not understood how upgrade unit to an other type (Paesant to Royal Guard as example)

have the unit to have level 5 ?

have I to upgrade Paesant first to Militia and Halberdiers before I could choose Royal Guard ?

Thank you

I have only an pdf as manual but there are nothing about it