Slitherine

How does one go about upgrading a unit? I saw some discussion that some units are supposed to be upgradeable.

So can anyone tell me which units are upgradeable and how you go about upgrading?

First, the unit must be in a city. You can only upgrade in a city hex.

Second, select the unit. In the lower left of the screen the unit and its factors will appear. One of the selections for the unit is a question mark (?). Click on this and you will be given the options for upgrading the unit. All options require credits.