1. Use it as is - it might be grainy feeling on your tongue, but it will measure and taste the same.
2. Warm it up - pour water hot from your kitchen tap into a pot. Put the jar of honey in the pot for a while and it will eventually liquefy. Don't heat the honey up, it can change the characteristics and damage some of the nutrients.
3. Put it in the sun - if you have a sunny window sill, you can put the honey in the sun and it will eventually liquefy. Sun exposure may darken your honey but it won't hurt it.
If you really don't want your honey to crystallize, consider keeping it in the freezer. Crystals form best at room temperature, so put a large container in the freezer and use it fill up a smaller container for daily use.
Keep your honey at room temperature so it doesn't absorb moisture. With proper care honey can last forever. That is if you can resist eating it!