A live flow is locked to protect customers who are mid-journey. The builder shows a green banner: Flow is LIVE - To edit your flow, add more steps, or make changes, you must first turn off the flow, the canvas is greyed out, and any attempt to add/remove a block returns Flow is Live!. Switch the header toggle to Off, make your changes, then switch it back On. Messages already queued for enrolled customers are not removed when you turn the flow off; only new enrollments stop.