Hi, I have the same router as you and I've reverted back to stock firmware plenty of times.
You'd have to SSH into your router, unfortunately, but I find this the easiest way.
- I use CMD for this: ssh root@192.168.1.1 then use your password.
- cd /tmp
- wget http://downloads.linksys.com/downloads/firmware/FW_WRT3200ACM_1.0.6.186168_prod.img
I use the sysupgrade command to actually revert back. For some reason, this is the only command that works for me. - sysupgrade -F cd/tmp/(name of the firmware).img
The -F forces the update.