To disable the mode, start with removing the ONLY_FULL_GROUP_BY from mysql console
mysql > SET GLOBAL sql_mode=(SELECT REPLACE(@@sql_mode,'ONLY_FULL_GROUP_BY',''));
After which, remove ONLY_FULL_GROUP_BY from phpmyadmin, then open phpmyadmin & select localhost. Then click on menu Variables & scroll down for sql mode. Finally, click on edit button to change the values & remove ONLY_FULL_GROUP_BY & click on save as as shown in the image below:-