If the unit is hardwired or just battery-operated, press the Test button. You don't need to hold it down. Press and release, and it should go back to normal "stand by" mode.
Smoke Alarms from Kidde
Properly installed and maintained, smoke alarms are one of the best and least expensive ways to provide an early warning when a fire begins. Smoke alarms save lives, prevent injuries and minimize property damage by alerting residents early to a fire hazard.