how can we change definition of a table once created?

hello sirs,,

can any one say how can we change the table definition once
created?

say, we have a table with 1000s of records. if one needs
to change the data type of particular column or if he
needs to change the width of a column what should be
done?

As of now, i am copying the data to a file(COPY COMMAND) and
creating a new table after deleting the old one and copying
the content of that file to the new table.

is it the only way to do it?

Bhuvan.