If you want to change the table default character set
to a new character set, use a statement like this:
ALTER TABLE tbl_name CONVERT TO CHARACTER SET charset_name;
So query will be:
ALTER TABLE tbl_name CONVERT TO CHARACTER SET utf8mb4;
Font: How to change the default charset of a MySQL table?
In PHPMyadmin:
You need to go to phpmyadmin > information_schema > schemata > change your DEFAULT_CHARACTER_SET_NAME with query
Font: Change default Charset