You can use gems to replace any active quest that you don't want to complete right now by simply tapping on the top-right green icon on each quest.

More specific steps are as follows:

Tap “QUEST” on the home screen and go to the quest section, then scroll down to “Active Quest” and tap the replace icon on the quest you don't want to finish right now. Then in the pop-up window, tap “CONFIRM” to continue (*Note: It will take you to store if you do not have enough gems) or tap “CANCEL” to return. It will cost you 120 gems for each quest change and there is no limitation for the quest replacement.

Basic rules

New quests refreshed will not duplicate with current quests.

New quests refreshed might have been completed before.

New quests' requirement is relevant to the remaining time of this quest season. 

Quest has been complete but failed to receive the rewards, the quest cannot be refreshed.