You can use the Reset button on the router as described above, or if you have web browser access to the 3200:
Luci > System/Backup Flash firmware > System Reset
Then SSH into 192.168.1.1 to perform the procedure to flash Linksys stock firmware.
The 3200 is a dual flash device. If you have only ever installed OpenWrt once to it, and Never upgraded OpenWrt, I believe you may be able to force it to boot up with stock firmware stored on the other partition by cycling the power switch in a certain sequence.
You need to use force parameter with sysupgrade in order to get it to accept the OEM image. (If you mean the original Linksys firmware)
"sysupgrade -F -n imagefilename"
And remember that after the router boots, it's IP address will be the OEM default. 192.168.1.1? 192.168.0.1? And you need to adjust the PC's own address accordingly. (The 10.10.10.10 has no meaning at that point.)
I think that with the ancient, deprecated, 18.06.x, you need to use SSH console (e.g. putty in Windows) to use sysupgrade with the "--force" or "-F" parameter. There is no force option in LuCI. (With 19.07 and master, that can be done from LuCI.)