Using Apple Watch with airpods and Siri whilst out running

Hello.

I need some advice please. When I go out running, I only take my Apple Watch (Which outside obviously isn’t connected to WiFi) and I also have my airpods in. I am able to listen to the music I have downloaded to the watch and I set this to start at the beginning of my run so I’m not messing with the watch whilst I’m running. However I would now like to know how I get Siri to work whilst I’m on my run if I wish to alter the music to something else that has been downloaded to the watch.

I want to able to tell Siri to “play........” but I don’t know how to make this work? Do I tap the airpods whilst I’m running and then speak? Or do I tap the watch and get Siri to work that way?

Im confused as I can’t seem to get Siri to work despite reading the support pages and following the instructions.

Thank you.