Boiling a can of condensed milk can be dangerous and messy, so please take all precautions.
Bring a large pan of water to a rolling boil. Make sure that you put enough water in the pan to completely cover the can you're about to cook.
Remove the label from the can of sweetened, condensed milk and carefully submerge it into the boiling water using a pair of tongs or a slotted spoon. Make sure you place the can on its side, so it can roll around. If you place the can bottom or top-side down, the boiling water can cause it to bounce up and down, which is really annoying...
Cook the can for 3 hours, making sure the can is covered with water at all times. Add more boiling water if necessary.
Using a pair of tongs, a fork or a slotted spoon, take the can out of the pan and place it onto a heatproof surface to cool. Make sure it has cooled to room temperature before you open the can.