I'd like explain the different alter table syntaxes - See the MySQL documentation
For adding/removing defaults on a column:
ALTER TABLE table_name
ALTER COLUMN col_name {SET DEFAULT literal | DROP DEFAULT}
For renaming a column, changing it's data type and optionally changing the column order:
ALTER TABLE table_name
CHANGE [COLUMN] old_col_name new_col_name column_definition
[FIRST|AFTER col_name]
For changing a column's data type and optionally changing the column order:
ALTER TABLE table_name
MODIFY [COLUMN] col_name column_definition
[FIRST | AFTER col_name]