How can I increase the difficulty to 200% in the Armageddon's Blade campaign?

In the video at your link, the player is playing through the ORIGINAL version of the campaign. In the remake, the bonuses are different, and the initial comments indicate what the player should expect.
I don't remember how to change the difficulty in the standard campaigns: someone wrote about it somewhere, but I forgot. I can't select it; there are no buttons to change it.