Diablo 3: How to Change Acts
In Diablo 3 for the PC, you can go back and play in any of the previous Acts you’ve already completed.
In order to change Acts, go to the character selection screen and click the Change Quest button on the left. You’ll see a list of all of your completed quests in your current Act that you can replay.
You’ll also see the difficulty option at the bottom, so you can go back to previous difficulties you’ve already completed and replay any of those quests as well.