How to alter a column and change the default value?

I got the following error while trying to alter a column’s data type and setting a new default value:

ALTER TABLE foobar_data ALTER COLUMN col VARCHAR(255) NOT NULL SET DEFAULT '{}';

ERROR 1064 (42000): You have an error in your SQL syntax; check the
manual that corresponds to your MySQL server version for the right
syntax to use near ‘VARCHAR(255) NOT NULL SET DEFAULT ‘{}” at line 1