Put the can (closed) in a pan. Cover with water and tap. Let it cook during 2 hours, verigying the water still covers all the can during all the process.
Let the can cool down out of water during 1 hour. Then, pour the dulce de leche in a sterilized jar or use it ;-)
Cooking time will be totally diferent if you use a pan or a pressure cooker. Cooking time in a pan : 2 hours Cooking time in a pressure cooker : 1 hour Cooking time in a cooker ( thermomix...) : 25 minutes.
Don't forget to remove the sticker if there is one.