After creating your Alpha, you can still change the video series. However, you can only choose ONE set of videos at a time because all the guides, handouts and materials will correspond to that specific video series.
1) To change the series, click on "Edit Alpha Details" at the top right side of your Alpha.
2) This will open up a menu with all the details you originally added. Click on the drop-down menu under "Series being used" and choose the video series that you want: