Reset iPhone's Frequently Used Emoji's in Emoji keyboard
The Frequently Used Emoji section in the iPhone's built-in Emoji keyboard can be reset to the default set by going to Settings app → General → Reset and tapping Reset Keyboard Dictionary.



This is the only approach to reset the frequently used Emoji list (other than manually entering the Emoji's from the default frequently used list as shown below to manually reset the list).



Note: The command is not specifically meant to reset the Frequently Used Emoji list (rather it's the side effect), you'll end up resetting your custom dictionary. Also, all the downloaded dictionaries will be deleted. (Hence, you will no longer be able to quickly see the definition of a word by selecting it and tapping Look Up). Dictionaries can be easily re-downloaded by going to Settings app → General → Dictionary and tapping on desired dictionaries.