Question: I made caramel with condensed milk one month ago. Now I want to use it again, but it has sugar granules on it. So what should I do to make it smooth again?

Answer: 
If the crystals have formed in the same way the crystals form in Honey, then you could try warming the caramel. Honey forms crystals over time, and these are removed by placing the honey in hot water for an hour (you will need to keep the water hot, above body temperature but not boiling).
Once crystals appear in dulce de leche you can't remove them (a far as I know).
But there are several methods to help not getting the crystals. Industry adds β-D-galactosidase. Adding lactose micro crystals, as they do in ice cream manufacturing, isn't practical due to technical limitations. 
[Effect of cooling temperature and formulation in Dulce de Leche manufacturing, SPANISH](http://bdigital.zamorano.edu/bitstream/11036/728/1/T2257.pdf)

For home manufacturing some options are using a thick iron pot that cools down slowly. If you open the can, you can add marbles to avoid it getting stuck to the bottom, never introduce spoons to stir it.