The Sell Defense icon allows players to sell a nearby Hero Defense in exchange for a reduced amount of mana. The amount of mana received depends on the Hero Defense's current health as a percentage of its total. It is accessed from the Action Wheel.
The amount of mana received is independent of any upgrades made.
If you sell a defense during the same Build Phase you bought it, you get a full refund. That allows you to fix a misplaced tower.