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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! 

I love this game!

It's so soothing, hopeful, and nostalgic all at once! Thank you so much for making it and sharing it with the world!

This saved me a lot of headache! Thank you!