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Once unlocked (on Regular-sized maps) there is a pair of buttons above the build menu, labelled "I" and "II" 

clicking these toggles between two different build menus. 

As for undo, I don't know if there is a way to do it, but I wish there was. 

Hope that helps! 

As far as I know Undo button only appear when you build a building that does absolute nothing. I would love to have it permanent there as I keep clicking thing by mistake all the time

I have the same issue, I can't click the "I" and "II" on Linux.