You can stop at any ticket window or call 802-253-3000. You can view a map of the base area here: https://www.stowe.com/the-mountain/about-the-mountain/trail-maps.aspx?tc_1=2
If you are using Mobile Pass or Ticket through the My Epic App, look for the green checkmark on your Mobile Pass or Ticket, indicating that your phone will scan from your pocket. Make sure to enable Bluetooth on your phone prior to scanning your Mobile Pass or Ticket in the lift line. More information on the My Epic app and Mobile Pass and Ticket is available here: https://www.epicpass.com/app.aspx