[acomjean]

You can heat it in a microwave to reverse the crystallization.

Do not do this ifs the honey is in a squeezable bear container. The honey will boil, make a hole in the bear and spray honey all over the inside of your microwave (the turntable helps this). This will make a huge mess and will make opening the microwave more challenging.

[yumraj]

I think the general recommendation is to put the bottle in warm/hot water. I don’t believe microwaving is a good idea, unless done at low power for longer.

[avidiax]

Sous-vide at 110F. It will take hours, but it won't affect the flavor.

[dredmorbius]

Warm water is sufficient, though with recent cautions about heating plastics and leaching of chemicals, I'd prefer transferring the honey to a glass jar if it's not packaged in such already.

You can double-boil if you want, where the jar sits in a shallow water bath which you boil for 10--20 minutes or so to decrystalise the honey.