It is rare to put everything into a database design correctly on the first try, so you need a way to go in and revise you design as you build the database
The SQL Command for this is the ALTER Command
There are 4 ALTER TABLE Arguments you want to be familiar with
ADD
DROP COLUMN
RENAME COLUMN
MODIFY
Used to add a new column (field) to the table
Used to modify an existing attribute of a field, for example type or size
The opposite of an add, used to remove a field from a table
Used to rename an existing field in your table