Sheet metal isn't all that different from a car's bodywork, so most methods which will remove dents from a car work for sheet metal. One involves using a blow-dryer or a paint stripper which emits heat to warm the metal of the dent. On a molecular level, the increased heat causes the molecules of the sheet metal to become excited. They vibrate at a slightly greater rate, making the sheet metal more pliable. In this state a rubber mat is placed over the dent and a mallet is used to beat the dent flat. Because the metal has been made more pliable, the mallet less likely to leave marks behind.
Dents in sheet metal are the result of the metal being stretched by something impacting with it. Chilling the metal, such as with freon or dry ice, does the opposite of exciting the molecules of the metal. It causes them to slow down and pack closer together, making the metal contract. This contraction causes the dent to pull back into shape with the rest of the sheet. One simply applies Freon or dry ice to the very center of the dent for a few seconds at minute long intervals until the dent disappears.
Very large dents can be fixed by actually pulling the dent into line with the rest of the sheet metal. Specially designed suction devices are sold in car repair stores for this purpose. One drills a hole through the very center of the dent and inserts the hook at the end of the suction device, which typically looks like a plunger. The plunger is pressed up around the exterior of the dent and locked in place so there's no air inside, and then the user pulls very hard. Once the dent is removed, the hole is patched. A circle of steel is fitted inside the hole and welded into place. Any remaining metal is removed with an angle grinder so the sheet is smooth and unblemished.