how to reset an airport express A1264?

Since you have a used AirPort Express, I would recommend that you perform a full Factory Default Reset to completely clear out all the old settings and profiles that might still be stored on the AirPort Express.

The "Hard Reset" that Kappy suggests will not always clear out all of the old settings....and it will not clear out any previous profiles that might be stored on the Express.

To perform a Factory Default Reset.....

Power off the Express for a few minutes

Hold in the reset button first, and keep holding it in for another 8-10 seconds while you simultaneously plug the Express back into power

Release the reset button after the hold period, and allow a full minute for the Express to restart to a slow, blinking amber light

The Express is now completely reset and can be configured with a new setup

On your Mac....

Open Macintosh HD > Applications > Utilities > AirPort Utility

Click on Other WiFi Devices

Click on AirPort Express

The setup "wizard" will open up, analyze the network, and suggest a setup based on what it sees. Follow the prompts.

On the backside of the AEX with respect to the AC plug you will find a Reset button you can depress with a toothpick. There will be two small circles, one on each side of the case centerline. The one on the right is the one you use to reset it. The device must be plugged into the wall outlet. Hold in the reset button until the status light starts blinking fast. You may need to do this more than once.

See: Resetting an AirPort base station FAQ

I have tried this several times with my A1264 and I either can't see it in the "Other Devices" or when I do see it, I get "An Unexpected Error Has Occurred". Is there a fix for this or is my Airport Express fried? I'm running Yosemite.

Apparently the "unexpected error" is related to some cryptic ipv6 settings. This answer here helped me: Airport Express Unexpected Error