Create or replace trigger postgres
Solution 1:
No way to create or replace a trigger but can do this way
DROP TRIGGER IF EXISTS yourtrigger_name on "yourschemaname"."yourtablename";
Solution 2:
Postgresql has transaction DDL so BEGIN > DROP > CREATE > COMMIT is the equivalent of CREATE OR REPLACE
This is a nice write-up of how postgre's transactional DDL compares to other systems (such as oracle)
Current postgres planned features regarding triggers do not include adding the REPLACE syntax.