King’s Bounty II involves a lot of fighting, and your troops will always be at risk of dying during the ferocious tactical combat. As they get whittled away, you will want to try and replace them when you can, or else you risk going into battles underpowered.
To replace your troops, you need to find a trader who will deal in combat contracts, and whom you can buy fighters from. These are marked on the map by a sword and helmet symbol. As long as you have the gold, and enough capacity to lead all the units, you can hire whomever the trader has available. Each trader has a different roster of troops that you can buy, so sometimes it will be a good idea to fast travel somewhere if you are looking for a specific troop type.
To buy troops, all you need to do is interact with the trader and buy them from the column on the right-hand side of the screen. Each unit will have a gold cost that you will need to pay.
If you already have a unit within your army that has lost some members, then you will be able to buy just the amount of members that you are missing for the relevant amount of gold. This will happen automatically if you just buy the available unit, you don’t need to do anything special or select a special option to make it happen.