Roll back firmware origin

I installed this binary file "openwrt-18.06.1-mvebu-cortexa9" on a linsys WRT 3200 ACM maitenat I wish to return to the orgine firmware.

How to do ? I tried to chage the binary but I have an error message: the file is not compatible.
I have exceuter orders online but without success.
Thank you for your help

openwrt-18.06.1-mvebu-cortexa9-linksys-wrt3200acm-squashfs-factory (1)

That is covered in its wiki page, rather convoluted and verbose, but it's there.