Older Apple wireless keyboard, cannot connect

I have an older Apple wireless keyboard, with AA batteries. I am trying to bring it back into service and connect it. When I turn it on, the green light turns on, not blinking, and goes off after 2-3 seconds. I understand this means it is still connected to another device.

I am sure I no longer have that other device. Is there a way for me to reset the old wireless keyboard so it can be paired with another device?