Just click on the reserve troop and then click on the slot where you want it.
You can replenish an existing stack or you can make entirely new stack providing you don't have any of those units in your troop.
If you mean how to buy units for the reserve - it is just bad design - you must buy units for your normal stack, then click on the reserve and transfer your normal units to the reserve. The good thing is you can do it as many times as you want.
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