Most honey granulates eventually and, depending on the source of the honey, it might take a month or it might take years. For example, Maple blossom honey stays liquid much longer than other types of honey. If your honey granulates, that does not mean it is going bad. Many people prefer granulated honey to liquid honey and in most cases granulated honey is soft enough to be spreadable.
If you still want to melt your honey, you can put it in hot water (maximum of 37C) to bring it back to liquid with minimal change in taste or aroma. If you put honey in the microwave or in boiling water you are almost certainly going to see some decrease in enzymes, flavour and aroma.