There are four stats to upgrade in your tank. Balance everything out before purchasing additional upgrade levels. This is because each particular upgrade will improve a particular stat of your tank while decreasing another one. For example, if you increase Attack, it will decrease the Aiming; if you increase the Defense, you will also increase the Reload Time. Because aiming tends to work well, it should not be a priority to upgrade. The recommended order to upgrade is Armor, Main Gun, Tech, Ammo, Reload, and Aiming; then repeating the order for the next cycle of upgrades.