how to alter an enum type

Hi,

I was wondering how can I alter an ENUM type? I have created a table
like this:

create type possible_answers as enum('yes', 'no');
create table questions ( question text, answers possible_answers);
insert into questions values ('Do you like me?', 'yes');

So my question is... How can I change "possible_answers" to enum('yes',
'no', 'maybe')?

I tried searching the documentation and mailing list, and I couldn't
figure this one out.

Thanks!