Hi,
The round nut has two 'lugs' on it that locate it into the bracket so it will not move, think square bolt in a square hole. You need to undo it using a spanner on the hex end off the bolt.
It may be stiff to turn as the bolt acts as a pivot for the pedals and they tend to seize onto the bolt/pivot so you will find you will need to do lots of wiggling, spray of CRC/WD40 type chemicals and maybe even some heat. On the plus side it is out of the Moke so you can do it all on the bench in a vice.
Terry