How do you Upgrade a Unit?

[Earthmichael]

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?

[azpops]

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.