ALTER SYNONYM v15

ALTER SYNONYM <syn_name> OWNER TO <new_owner>

-- create user t1 and t2
edb=# create user t1;
CREATE ROLE
edb=# create user t2;
CREATE ROLE

-- grant all the privileges to user t1 and t2 on public schema
edb=# GRANT ALL PRIVILEGES ON SCHEMA public TO GROUP t1;
GRANT
edb=# GRANT ALL PRIVILEGES ON SCHEMA public TO GROUP t2;
GRANT

-- connect to the database as user t1 and create the table
edb=# \c - t1
You are now connected to database "edb" as user "t1".
edb=> create table t(n int); 
CREATE TABLE

-- create a synonym for table t
edb=> create public synonym x for  t;
CREATE SYNONYM

-- check the owner of the synonym x is t1
edb=# select * from all_synonyms;
 owner | schema_name | objid | synonym_name | table_owner | table_schema_name | table_name | db_link 
-------+-------------+-------+--------------+-------------+-------------------+------------+---------
 T1    | PUBLIC      | 16390 | X            | T1          | PUBLIC            | T          | 

-- connect the database as user edb and change the owner of the synonym x to user t2
edb=> \c - edb
You are now connected to database "edb" as user "edb".
edb=# alter synonym x owner to t2;

-- check the owner of the synonym x is changed to t2
edb=# select * from all_synonyms;
 owner | schema_name | objid | synonym_name | table_owner | table_schema_name | table_name | db_link 
-------+-------------+-------+--------------+-------------+-------------------+------------+---------
 T2    | PUBLIC      | 16390 | X            | T1          | PUBLIC            | T          |