You can specify the columns that you want to update as long as you have the primary-key constraint columns available.
> create table t(pk varchar not null primary key, col1 integer, col2 integer, col3 integer);
> upsert into t(pk, col2) values('a', 2);
When you provide fewer columns than the schema defines, the other columns will receive no new value. This is an equivalent operation to an "UPDATE" in a traditional RDBMS.