Postgres update a table (only) if it exists

You need a pl/pgsql block to do the IF:

DO $$                  
    BEGIN 
        IF EXISTS
            ( SELECT 1
              FROM   information_schema.tables 
              WHERE  table_schema = 'public'
              AND    table_name = 'recipes'
            )
        THEN
            UPDATE recipes 
            SET lock = NULL
            WHERE lock IS NOT NULL ;
        END IF ;
    END
   $$ ;