I hear that there are 3 "words" you can learn for each shout that upgrades them, but I havnt found out how. So far I have 2 shouts and have killed 4 dragons, so I would like to use some of my souls to upgrade the ones I already have. Any help is very appreciated!
Highlight the one you want to upgrade, and if you have the second 'word' then press R on it. It should use a soul and upgrade. But mind you, you have to have the second word before you can 'upgrade' the shout.
You find the words at other locations. For example, for the Fire Breath there are 3 dragon words in the world to find. Though I have found the word for throwing your voice with all 3 words. But I had to deal with a freaking powerful undead priest of some sorts to get it.