I haven't found the option to.
Good news everyone! (Cue Professor Farnsworth voiceover). The first big Civ V patch should be arriving sometime next week. As listed on the official forums here
Start Original Answer:
Buildings are not currently sellable in civilization V. The only way to remove a building from a city is to have that city be conquered (which isn't generally a viable option).
It is hoped that there will eventually be a patch to allow for this, if for nothing else than to nuke annexed puppet-states useless barracks, armory, etc.
The patch is now available. You can sell a building by clicking on it in the right hand side of the city screen. You receive 10% of the buildings production cost in gold, but more importantly you eliminate its maintenance cost. You can only sell one building per city per turn. You cannot sell buildings in puppet cities until you annex them.