Which button do you press on the backup UHF control head to change from channel mode to manual frequencies?

The correct answer is the small red channel mode select button. Pressing this button allows the user to switch from channel mode, which typically allows for quick access to stored channels, to manual frequency entry mode. This is crucial for situations where predefined channel selections may not suit operational needs, enabling the crew to manually enter specific frequencies when necessary.

The other options do not serve this function: the large blue frequency knob is generally used for tuning to a specific frequency rather than switching modes, the green preset button is associated with selecting preset frequencies, and the yellow emergency switch is usually intended for emergency communications rather than mode changes. Understanding these distinctions helps in swiftly navigating radio functionality during critical flight operations.