alter index WITH ( storage_parameter = value [, ... ] ) for partition index.

postgres=# create table part(a int) PARTITION BY RANGE(a);
CREATE TABLE
postgres=# create table part_p partition of part for values from (minvalue) to (maxvalue);
CREATE TABLE
postgres=# create index part_idx on part(a) with (fillfactor = '14');
CREATE INDEX
postgres=# \d part
                Table "public.part"
 Column |  Type   | Collation | Nullable | Default
--------+---------+-----------+----------+---------
 a      | integer |           |          |
Partition key: RANGE (a)
Indexes:
    "part_idx" btree (a) WITH (fillfactor='14')
Number of partitions: 1 (Use \d+ to list them.)

postgres=# \d part_p
               Table "public.part_p"
 Column |  Type   | Collation | Nullable | Default
--------+---------+-----------+----------+---------
 a      | integer |           |          |
Partition of: part FOR VALUES FROM (MINVALUE) TO (MAXVALUE)
Indexes:
    "part_p_a_idx" btree (a) WITH (fillfactor='14')

postgres=# alter index part_idx reset (fillfactor);
ERROR:  "part_idx" is not a table, view, materialized view, or index
postgres=# alter index part_p_a_idx reset (fillfactor);
ALTER INDEX
postgres=# \d+ part
                                   Table "public.part"
 Column |  Type   | Collation | Nullable | Default | Storage | Stats target | Description
--------+---------+-----------+----------+---------+---------+--------------+-------------
 a      | integer |           |          |         | plain   |              |
Partition key: RANGE (a)
Indexes:
    "part_idx" btree (a) WITH (fillfactor='14')
Partitions: part_p FOR VALUES FROM (MINVALUE) TO (MAXVALUE)

postgres=# \d part_p
               Table "public.part_p"
 Column |  Type   | Collation | Nullable | Default
--------+---------+-----------+----------+---------
 a      | integer |           |          |
Partition of: part FOR VALUES FROM (MINVALUE) TO (MAXVALUE)
Indexes:
    "part_p_a_idx" btree (a)