ALTER TABLE <tblName> MODIFY <columnName> dataType constraint;
Para sua exigência será
ALTER TABLE <tblName> MODIFY <columnName> datetime;
Consulte http://dev.mysql.com/doc/ refman/5.1/en/alter-table.html
ALTER TABLE <tblName> MODIFY <columnName> dataType constraint;
ALTER TABLE <tblName> MODIFY <columnName> datetime;