That’s the bottom tab in the research academy, each troop type has 5 tiers.
I suggest holding off on this until, you know want you want to use to attack and defend.
Upgrading them will drastically increase the training time, resources and healing time.
Once you do a troop tier upgrade, all of your existing troops of that type (ie. human infantry) will be converted to the higher tier and you will lose between 30-50% of your current lower tier troops. It is best to train a bunch before upgrading as the higher tiers cost a lot of gold and rare resources.
Here’s the estimate resources and time to Train and Heal 1,000 human troops:
T1 – 100k resources in 3 hours, Heal in 10 seconds
T2 – 200k resources in 6 hours, Heal in 16 minutes
T3 – 600k resources in 12 hours, Heal in 33 minutes
It is very important to keep 1 type of troop as T1 forever, and it’s recommended to use the human Basic Catapult for this. It’s the cheapest troop to train and best for gathering. During alliance wars and kill events your gathering troops may be attacked, so this will minimize the damage done when it happens. If you have already upgraded the Basic Catapult on tier 2, then keep one of the other catapults at level 1.
The way defending works in this game is your all your troops in your castle becomes your highest tier (strongest) troops. For example, even if you have 1 troop, say a T5 Infantry Orc, and 1 million T1 catapults; all of those catapults changes to T5. This happens when you are defending attacks and joining any rallies.
To defend against all attacks in this game, you just have to have enough T1 troops of any type. If you accomplish this, you will never need to shield or worry about getting attacked.