Comments for schemas and tables may be updated using SQL DDL statement  COMMENT ON.

Comments for columns are updated using:

ALTER TABLE <table-name> ALTER COLUMN <column-name> COMMENT '<comment-text>';

The <column-name> for nested STRUCTS follows simple dotted notation. For example, a STRUCT for a person's physical address:

ALTER TABLE person ALTER COLUMN physical_address.postal_code
COMMENT 'Postal code for the person\'s physical address.';

Less obvious, the <column-name> for fields in ARRAY<STRUCT<>> requires additional keyword ".element". For example, an array of address STRUCTs that includes an "address use code" field:

ALTER TABLE person ALTER COLUMN address.element.use_code
COMMENT 'The address use code indicates either \'physical\' or \'mailing\' address.';

As of this writing, each ALTER COLUMN ... COMMENT requires an individual ALTER TABLE statement. The column comments cannot be chained together as a list of as clauses within ALTER TABLE.