Removing NOT NULL Contraint

I may be missing something simple here, but I have an attribute in a
table that was created as NOT NULL. I now need to remove this
constraint. I don't see a proper way to do this. I think I can update
the pg_attribute table and change attnotnull from true to false. Is
this an acceptable solution or is there a good reason that I should not
directly edit the system tables?

Maybe I'm missing something but I didn't see another way to do it other
then creating a new table, selecting into it and dropping the old table,
which seems like a lot of work for something so simple.

Thanks,

Matt