Check the current owner of the tables using the below query where schema_name is the name of the schema in lower case.
select tablename, tableowner from pg_tables where schemaname='schema_name';
Generate sql statements to change the owner of all the tables using below command where new_owner is the name of the new owner and schema_name is the name of the schema.
select 'Alter table '||t.schemaname||'.'||t.tablename ||' owner to new_owner;' from pg_tables t where schemaname='schema_name';
Execute the output of the above sql command to change owner of all tables.
Verify the change by running the select command again.
select tablename, tableowner from pg_tables where schemaname='schema_name';