Tap the channel badge at the bottom of the screen to show the channel menu. From the menu the channel can be enabled/disabled and the channel voltage range(s) can be configured.
Tapping and holding the channel badge will switch the channel on or off.
If the Oscilloscope front end can measure voltage ranges different to the default 0-3.3V range of the Pico (eg if you have added a resistor divider in front of the ADC pin) then you will need to configure the channel as described here in order for Scoppy to display the correct voltage values.
The probe attenuation value can also be configured. Tap the channel badge and select Settings and then Probe from the menu.