Postgres 13

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CREATE TABLE table1 ("ID" int, "TXT" text);
INSERT INTO table1 VALUES (1,'foo');

CREATE TABLE

INSERT 0 1

DO $$
DECLARE row record;
BEGIN
  FOR row IN SELECT table_schema,table_name,column_name
 			 FROM information_schema.columns
			 WHERE table_schema = 'public' AND 
     		               table_name   = 'table1'
  LOOP
    EXECUTE format('ALTER TABLE %I.%I RENAME COLUMN %I TO %I',
	  row.table_schema,row.table_name,row.column_name,lower(row.column_name));	
  END LOOP;
END $$;

DO

SELECT "id","txt" FROM table1;

id
txt

1
foo

SELECT 1