I'm referring this example flow. I've in one of my flows, a client_secret as payload. I'm trying to store this payload as an environmental variable. I'm a newbie and I'm not sure how to do it. Can someone help me please?
You can not store data in a flow into an environmental variable. You create the Environmental variable in your settings file and store a constant in it. Then in your flw you can access that value.
Just to expand on that, environment variables exist in the host OS. They are only read when starting Node-RED so even if you were to update them, they wouldn't help you.