How do I update a table using a staging table to avoid inserting duplicate records?

I have a main table called arrests with primary key arrest_key that is currently empty. I have a staging table called arrests_temp that contains one month of data. The staging table looks like this: https://i.imgur.com/v8QGynD.png

I have the following so far:

UPDATE arrests 
SET arrests.arrest_key = arrests_temp.arrest_key 
FROM arrests LEFT JOIN arrests_temp 
ON arrests.arrest_key = arrests_temp.arrest_key 
WHERE arrests.arrest_key != arrests_temp.arrest_key;
    
INSERT INTO arrests (arrest_key, arrest_date, pd_cd, pd_desc, ky_cd) 
SELECT arrest_key, arrest_date, pd_cd, pd_desc, ky_cd
FROM arrests_temp 
WHERE arrest_key NOT IN (SELECT arrest_key FROM arrests);

I keep getting the following error:

ProgrammingError: (pymysql.err.ProgrammingError) (1064, "You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'FROM arrests LEFT JOIN arrests_temp \n ON arrests.arrest_key = arrests_temp.ar' at line 3")

[SQL: 
    UPDATE arrests 
    SET arrests.arrest_key = arrests_temp.arrest_key 
    FROM arrests LEFT JOIN arrests_temp 
    ON arrests.arrest_key = arrests_temp.arrest_key 
    WHERE arrests.arrest_key != arrests_temp.arrest_key;
    ]
(Background on this error at: http://sqlalche.me/e/13/f405)

What am I doing wrong?