Upgrade for City Units etc?

[targul]

I just received a star in my game indicating I can upgrade. It does not say what or how. I have clicked the star and tried every city and unit without success.

Can someone tell me how and what you upgrade?

Sorry my manual has not arrived yet and the downloaded one will not open.

Thanks

[JudgeDredd]

I'm not totally sure how to use this either...but when I have clicked on it the cursor turns into a funny cross hairs icon and units show a number/number like 3/4 or 4/4. The 3/4 can be updated, the 4/4 cannot I think. I've never actually upgraded anything because I have not had enough resources!

[Montbrun]

If the unit is eligible to upgrade, the numbers will be red. Also, the unit cannot have performed any other function during the turn, has to be in port (for naval units), cannot have moved, not in an enemy ZOC, etc.

[Twotribes]

With the button clicked all units will have 2 numbers , like this 0/0 or 1/2 , the first number is the level it is at with that tech tree ( can be one of 3 things or more in some cases) the second number is the Tech level you raise it to.

There are 5 techs with 3 areas each. You may have researched something that you have no current units to upgrade.