1. Click on the Wi-Fi icon in menu bar > Open Network Preferences... or > System Preferences... > Network > Wi-Fi
2. Click Advanced... button
3. In the Preferred Networks: list, click on the xfinity listing
4. Click the Minus (-) just below the list, then click OK
You can also click and drag the networks in the list to reorder them - your most frequently used network should be at the top of the list.