MySQL - Adding or removing individual SQL modes

Adding only one mode to sql_mode without removing existing ones:

SET sql_mode=(SELECT CONCAT(@@sql_mode,',<mode_to_add>'));

Removing only a specific mode from sql_mode without removing others:

SET sql_mode=(SELECT REPLACE(@@sql_mode,'<mode_to_remove>',''));

In my case I wanted to remove only ONLY_FULL_GROUP_BY mode

SET sql_mode=(SELECT REPLACE(@@sql_mode, 'ONLY_FULL_GROUP_BY', ''));

More information about sql_mode https://dev.mysql.com/doc/refman/5.7/en/sql-mode.html