I'm not sure if you can change the stats and/or equipment on soldiers in the middle of a campaign though. You may have to start a new one.
When you open up the troop editor, there will be an item pool for each unit. The reason there is more than one of each type of item (e.g. there might be 3 body armor pieces) in the item pool for each unit is so that they don't all look exactly the same. Each unit will start out with a random one of those pieces. If you want all of a specific type of soldier to have a specific item, make sure it's the only one of that item type in the unit pool. For example, if you want all of your knights to be wearing Coat of Plates, make sure it is the only body armor item in the item pool for your knights.
Whose property is my body? Probably mine. I so regard it. If I experiment with it, who must be answerable? I, not the State.