If you want switch from Ethernet to Wi-Fi, first remove the Ethernet cable from the Tablo, and then reboot the Tablo. To reboot your Tablo, press and quickly release the blue reset button on the back right of the box.
The Tablo will then boot into Wi-Fi setup mode. Instructions for this process can be found here.
If you want to switch from Wi-Fi to Ethernet, attach the Ethernet cable to your router, then Tablo. Next, tap the blue reset button on the back of the Tablo once. Wait for the blue LED to stop flashing. Once the blue LED is solid, your Tablo is now online and using the Ethernet connection.