How to replace postgresql function body?

Yes, you can update the definition of the function using the Postgres CREATE OR REPLACE FUNCTION syntax described in the documentation for CREATE FUNCTION.

So if you've got a function you could replace it by re-declaring it. For instance, here's how I used this to replace id_generator after a schema change:

ALTER SCHEMA public RENAME TO app;

CREATE OR REPLACE FUNCTION app.id_generator(OUT result bigint) RETURNS bigint
  LANGUAGE plpgsql
  AS $$
    DECLARE
        our_epoch bigint := 1111111111111;
        seq_id bigint;
        now_millis bigint;
        -- the id of this DB shard, must be set for each
        -- schema shard you have - you could pass this as a parameter too
        shard_id int := 1;
    BEGIN
        SELECT nextval('app.global_id_sequence') % 1024 INTO seq_id;
        SELECT FLOOR(EXTRACT(EPOCH FROM clock_timestamp()) * 1000) INTO now_millis;
        result := (now_millis - our_epoch) << 23;
        result := result | (shard_id << 10);
        result := result | (seq_id);
    END;
  $$;

The result of which changed the function in place without needing to update tables that relied on the function.