Changing columns

Not sure how to do this, but it shouldn't be too ugly. I want to drop
columns from a table as well as be able to change the data type of a column.
Where should I be looking in the source for altering tables? For removing I
don't imagine it could be too ugly as all it should be is a reverse add.
Not so certian about changing the data type though. Can this be done simply
by adjusting something in pg_attribute or am I missing something? Also, for
the future, MS Sql already has this and it'd be very helpful if pgsql did:
modifying data between databases using a select, insert, update, or delete
(execuse me if I'm wrong in my syntax, but in MS Sql I believe it
dbname..tablename.column).

Geoff