Hi Tammy,
You can do it by using the WAAS recovery CD.
Just boot the 674 with the CD inserted and select option 2.
This will reformat the flash.
More info: http://www.cisco.com/c/en/us/td/docs/app_ntwk_services/waas/wae/troubleshooting/recoverycd/guide/17088rcd.html
Regards,
Nicolas