If you have an item that increases the EXP of a certain unit category (for example, Arbalest - Archery EXP +30), equip it on a person with the corresponding unit category. After doing so, choose a certain unit type with the higher EXP points. After you have selected one, you can unequip the item, and have the new class with your basic EXP.
One of the obstacles in the game is having a strong force on one side of the map while needing that force on the other side. Since the distance per turn rules bar any army from getting across the map in one turn, this can lead to a second rate army having to fight a first rate army. If the two distant armies are connected by territory (linked to the capital), you can simply trade one army for another by using the following trick. Choose "Form" at one army and remove all the generals and strategist (swap the commander for a different one). Then, swap those free generals and the commander for the generals on the weaker army. Another benefit of this is that later in the game, when you want to expand, you can set up armies with only a commander (smaller armies allow for more armies), and send them out toward cities you want to take. If you think that you might have to fight a battle to take a city (if it is not empty), use the general swap trick for an instant army.