how do you upgrade units in civilization 5?

Question

how do I upgrade my units in civilization V?

Answer 1

you'll get an alert when a unit is ready to be upgraded and you can select the upgrade, you may have to wait a couple turns to get an alert

Answer 2

Once you have researched a better unit, a new button will appear in the 'lesser' unit's taskbar. If you have enough gold you can click it and it'll upgrade. However, this will cost the unit its turn. To maximize its potential, move it (don't attack) and then upgrade right before your final tile.

Answer 3

Have them kill stuff to get Xp, or build a barrack where they are being made to give a boost of XP.