To upgrade a unit, you need the right technology and enough gold to upgrade. The unit must also be in your territory.
Once the right technology is researched, the upgrade button will appear. This will replace the unit with a better, more modern one with the same experience and promotions.
Pop Culture ReferenceEdit
Harder, Better, Faster, Stronger is the name of a song by Daft Punk
This achievement is referred as ACHIEVEMENT_UNIT_UPGRADE in the debug file.