Linksy E8450 install on fimware 1.2.00.360516

Hey look I'm not by any means a professional at tech but I'm willing to give anything ago I've successfully installed openwrt on an Asus via ssd
Now the problem I'm having here is it rejecting the imagine so Im guessing the only way is to downgrade the firmware is that done just on the Linksys GUI or is there more to it and I can't seem to find a file to download that will downgrade my firmware amd i don't wanna just start uploading everyone I see n brick it
Or is downgrade done through another method
Any help would be very appreciated im kinda stuck lol

Ok thanks mate I'll give that a go when I get home