How do insert data into a table that already exists?

I'm trying to insert data into a table that already exists, but I cant find anything on how to do this. I only found how to insert this data into a new table.

Syntax error at or near Insert

Tutorial I visited

 SELECT film_category.film_id, film_category.category_id, rental_duration, rental_rate
 INSERT INTO category_description
 FROM film_category
 LEFT JOIN FILM
 ON film_category.film_id = film.film_id

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