Now you have the opportunity to make your army even stronger by applying new upgrades to your units. The Veteran upgrades are available only for units which have been upgraded to their maximum level. Then you should decide which aspect of your units you want to improve - their attack or their defense.
Note that only one of the 2 Veteran upgrades can be applied to each unit type, so choose carefully which ones you will apply and to which units.
You can use this option by paying 200 Credits and waiting a few hours or by paying with Resources and waiting longer for the upgrade to complete.