update mysql.user set authentication_string=password('xxxxxxxx') where user='root' and Host ='localhost';
flush privileges;
quit;

You can try to disable the only_full_group_by setting by executing the following:

mysql> set global sql_mode='STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION';

mysql> set session sql_mode='STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION';

mysqlcheck -uroot -p  --auto-repair --optimize --all-databases