column insert/alter got me stumped!

guys,
just wanna change value of 1 existing column

# insert into tablename (columnname) values ('value');
...works

# select columnname from tablename where columnname='value';
...works

# insert into tablename (columnname) values ('value') select
columnname from tablename where columnname='value';
or
# insert into tablename (columnname) values ('value') where columnname='value';
...combinations don't work

# alter table tablename alter column columnname set value='value';
...doesn't work either

embarrassingly simple? pgadmin III thinks so
jzs
http://www.postgresql.org/docs/8.1/static/sql-insert.html
http://www.postgresql.org/docs/8.0/static/sql-altertable.html