Originally Posted by Sensei Warrior
No it is not a mod, it is a normal function of the game. No worries when I first started playing I didn't know it could be done either. Anyways onto how to do it.
First you need a unit you want to repair/retrain. For this exercise we will say it is a unit of Spearmen that has been reduced to 1/2 its number from a backstabbing attack by the unhonorable Argonese.
Next the Spearmen need to be in a Provence where Spearmen can be made. By in a Provence I mean in it at the start of the year not moving to it. If you have to move to it you'll be able to do this on the year after the piece is moved to the Provence.
Then left click on the train unit button for the Provence. It is to the right of the Building button in the lower left hand side of the screen.
Left click on the stack that has the injured unit. Go down to the bottom of the screen where it lists all the units of the stack.
Left click and hold on the icon of your injured spear unit. While holding the button drag it over to the train unit window and set it in one of the empty queue boxes at the bottom of the screen. You'll notice the unit disappears from the stack and appears in the training queue with numbers telling you how many men and how much valour, like they do in the battle screen.
When you click the end year button and go to the next year you'll see the unit has been replenished back up to its full number.
You can do the same exact thing for units that are injured or full, but are in a provence that has an armor/weapon upgrade (has an armorer or metalsmith in it). When the unit is 'retrained' it will come out with an armor/weapon symbol on its icon noting that the unit has upgraded armor or weapons.
Once you figure out how to do it you'll know when to do it. It opens up a whole series of strategies for you to think about. Good luck and by the way Welcome to the Org