On Large tables dropping a column is time consuming as it needs to work on lot of records.To prevent this you can perform a logical delete of the column by Using the SET UNUSED.

ALTER TABLE table_name SET UNUSED (column_name);
ALTER TABLE table_name SET UNUSED (column_name1, column_name2);  

Once this is done the column wont be visible to users.

Later you can run a query to delete unused columns from tables

ALTER TABLE table_name DROP UNUSED COLUMNS;

In case you are working with table with less number of rows we can use drop command to drop the table column which performs Physical delete rather than using SET UNUSED which drops the column later

ALTER TABLE table_name DROP COLUMN column_name;
ALTER TABLE table_name DROP (column_name1, column_name2);

Alter table to change Column Type

ALTER TABLE emp MODIFY ename varchar2(50);