I personally recommend to warm your cherry pit pillow with your heating because it is the most gentle way. So you get sure the pillow will remain as new as long as possible. Especially microwaving is a pretty aggressive way, but if you only have a microwave, it's okay, the cherry pit pillow is pretty robust.

Oven
Preheat oven to 100°C (212°F)
Put cherry pit pillow into the oven for 5-10 minutes.
Do not leave the oven unattended.

Microwave
Microwave the pillow at 400 watt.
Only microwave it for 10-20 seconds.
Repeat that until the pillow is hot enough.

Heating
Just put the pillow onto your heating.

Fridge
Just put the pillow into your fridge.
As soon as you need a cool pillow, take it out.

How to clean your cherry pit pillow
If you use the cherry pit pillow often, it surely will get dirty from time to time. The pillows are very resistant, however I suggest you clean them this way.

Take a bowl with hot water and put some fabric softener into the water.
Put your cherry pit pillow inside the water for 15-30 minutes.
Remove dirt by washing it with your hands.
Remove fabric softener by washing in clean water.
Let it try.

Have fun with your cherry pit pillows!