You sell it off. At the bottom of the screen there's a $ (dollar sign) button. Click there, then hover over the building you want to sell. The pop up will tell you the resources/population/culture you'll get back from selling the building. Click on the building (still in 'sell' mode) and another bigger box pops up telling you what you get back and letting you confirm you wish to sell it. Click there and it's gone. For roads, you don't get anything back so no pop up when you hover, but you still click on the road square and it's gone. There will be no warning box, so make sure you're not deleting a 'good' road square instead of the one you want to delete. If you just wish to replace a road (say a simple path with a 'better' road square that gives culture but costs resources), you don't have to sell it off first, you can just build the new road square over the old one, click to build and it pops into place. Making sure you're selling off the right building sounds simple, but double check everything. My first thought when someone said they 'sold' a building by mistake, was 'how could you do that?' A few weeks later I sold off a lvl 17 residence instead of a lvl 1 temporary factory because I wasn't paying attention to the warning boxes...