First of all, anyone who posts here - please make a new topic with your question. That way other people who might be wondering the same thing will see it at once.
Second, there's really no need to do what you are asking. The cards are already arranged in the desired order so you will study all of level 1 before starting level 2 and so on. Just make sure you have the setting "show new cards in the order added" and not in a random order.
But if you really want to do it, here's one way:
1) Open the card browser.
2) Click on the deck name in the left column and then click Ctrl+A in order to select all the cards in the deck.
3) Click Suspend (it's in the top row). The numbers in the Due column will turn from "x" to "(x)".
4) Find "dif1" on the left side, click on it to show all the cards in level 1, press Ctrl+A again and click Suspend again. This will unsuspend all the level 1 cards.
5) Repeat step 4 for level 2.