I am a bit confused on upgrading the Dice Pool with Destiny Points (and other means). When it says to upgrade a Dice Pool one step, does that mean add 1 yellow in place of 1 green die (or red in place of purple)? Is one step altering one die in the total pool?
Thanks.
Yes, basically.
Upgrading does one of two things:
1) If there is at least one green die in the pool, switch a green die to a yellow
2) If there are only yellow dice in the pool, add a green die.
Upgrading by steps means to repeat that process that many steps.
So Upgrading YYY by two steps means going:
YYY -> YYYG -> YYYY
Hi, there are two terms the book uses that are very specific. Increasing a difficulty means adding a purple die. Upgrading a difficulty means switching a purple to a red.
The same goes for ability dice, but usually the base pool is already decided by the character's characteristics and skill ranks, so it is very rarely increased. Upgrading a positive dice pool means switching a green to a yellow.
Edit: beat by Litheon, but with added term definitions 
Great, thanks guys. I was 75% sure that this is what the book meant, but didn't feel it was clear enough to be certain. Big help. 
Can you please explain the subject further?
Assume the pool dice is YYGG.
What happens if I need to upgrade dice pool?
Does it become YYYG, or YYYY?
Is upgrading dice pool the same as upgrading dice?
In my opinnion it isn't but I'm fairly new to this and trying to sort it out.
To me, it looks like in above scenerio, it should be:
-when upgrading dice: YYGG->YYYG
- when upgrading dice pool: YYGG -> YYYY.
It would be YYYG. The wording might be bad, but in this case, you upgrade once. So you turn a green to a yellow, or if there are no greens, you add one.
Upgrading the dice and upgrading the pool mean the same thing: you take a green and change to yellow (or purple to red); or if you don't have any greens (or purples) you add one.
Upgrading also happens in steps all the time. If something requires you to upgrade 4 times, you would change green to yellow 1 at a time until you had all yellow. Then you would add a green. Then, if you still needed to upgrade, the green you just added would change to yellow.