Press the "Test/Silence" button when the device sounds.
Remove the alarm from the wall or ceiling.
Clean it with a soft, clean cloth. You can also vacuum it using a soft brush attachment, but don't overreach on a ladder. You may find it easier to take the alarm down
Re-install the alarm and see if that fixes the trouble. If you continue to get false alarms, relocate the alarm. The alarm should be position more than 4 inches from a ceiling/wall join.
Replace the detector if you continue to get false alarms after relocating it.