Press <F2> to open the "War Ministry" page, which is divided between showing the Units currently being trained on the left and details of such Units, if you click on one, on the right. Above the right side is an icon with two soldier busts. Click on this to open the Replacements menu.
Here you will see 5 icons --a soldier, a cavalryman, a cannon, a supply wagon and a ship, and above those an icon with "+1* on it. Click the "+1" icon to switch between ordering or canceling 1 or 5 replacements per click. Left-click the appropriate icon to add +1/+5 to your order of infantry, cavalry, artillery, supply unit/support unit, or naval replacements. Right-click to reduce your order by -1/-5.
During turn execution the replacements will be added to the appropriate replacement pool and can already be used during the beginning of that turn. So if you are missing hits from a type of element and order replacements, after turn execution you might not see all of the replacements you ordered, as some of them have already been used to replace hits to units on the map.