You can upgrade your units to increase their battle stats.
You can see what stats will be upgraded by clicking on the Information button at each unit. Some stats do not depend on the unit's level (e.g. Range or Damage type), but you can make your warriors invincible by upgrading their Damage and Armor.
Some upgrades may require you to build or upgrade the Tech Lab or Space Lab, and some may need specific Barracks upgrades.
To upgrade a unit, you will need:
1. To build and upgrade a certain type of building.
2. The unit should have enough experience.
To start upgrades, you must first build a Barracks (note that some unit upgrades may also require a Tech Lab or Space Lab). Once your Barracks are built, visit them and click the 'Explore' button to select which unit to upgrade. There is a small red progress bar over the unit pictures. It shows the experience the unit has already gained at its current level. If it is full, clicking on the unit will open the upgrade window. If not, you can go to the 'Retrain' tab and transfer some experience from other troops to the unit you want to upgrade, or buy the experience you need from Lee The Merchant. Once you have enough experience, go back to the 'Upgrade' tab and start the upgrade.
The second condition for upgrading a unit is its XP. Do not confuse your XP for a unit's XP. Each unit has its own XP scale. They earn XP in battle. The more damage they deal, the more XP they earn. The XP is shown in the de-briefing window below the icon of each unit. Upon earning the needed amount of XP points, the unit is ready for upgrade. Make sure to upgrade units in time, because they stop earning XP upon reaching the maximum limit. Also remember that units that have been upgraded to maximum level keep earning experience and use it to train other units.
Battles are not the only way to earn XP. You can open the Re-training tab and transfer XP from one unit to another. It costs some resources, but it allows you to upgrade units that haven't been to battle yet. Put a unit that will give its XP into the left slot, and the unit that will receive XP into the right one. Select the needed amount of XP and click on the payment button.
Process of upgrading
Once the unit has earned the needed amount of XP and you have the required building, you can start the upgrade. The process may take from a few minutes, but you can always finish it instantly for a certain amount of diamonds. While the Barracks, the Tech Lab or the Space Lab is being upgraded, you cannot start upgrading your units. Similarly, when you upgrade units, you cannot upgrade the buildings. Upon completing the process of upgrading of a building, all units of its type, both trained and those being trained, automatically get a new level.
You can also upgrade ammunition using Barracks, Tech Lab and Space Lab. Just choose an ammunition tab in the list of troops, and select the ammunition you wish to upgrade. Ammunition does not need any experience to be upgraded.