I had the same problem and found this site with a pdf containing instructions to change the combo. The pdf is pretty blurry, but I was able to still follow it to change the combination of my old supra.

http://www.realestatesupercenter.com/catalog/supra---combina…

Good evening Angela,
I just found a website that take in old lockbox as a trade in and use the credit to get a new lockbox. Hope this be another solution for your need.

Good Evening Angela,

I just googled Supra combination lock boxes and the one site I opened had directions on it for several different styles. If this link doesn't have what you need, just go back to google.

http://www.wholesalelocks.com/supra-lock-box-pushbutton.html

Hope this helps - See you in the office.