To grow a pet, you'll need a special resource - Lu'mian.
 

Where to find  Lu'mian?

Lu'mian can be found:
- On expedition islands
- On Endless Islands
- In a temporary adventure (if any)

How to upgrade a pet?

The older a Pet is, the more often it can use its ability and the better resources it will be able to find during adventures with you.

To upgrade your pets, use the new Lu'mian. Open the Nursery, select the pet you want to upgrade, tap on the level above and then tap the "Upgrade" button.  You will need to upgrade your pet 5 times before it is fully grown.