if-statement: MySQL - IF Query with STR_TO

Greetings Stackian Overflowers,

I'm trying to overwrite data in an existing column and do a STR_TO_DATE in the case it is already set.

Here's the code :

SELECT IF(columnA = '../../..' , '0000/00/00' , STR_TO_DATE(columnA, '%m/%d/%Y) 
FROM tablename
WHERE extract_date = '2018-12-31';

In all reality, I must use this logic to make an update on the table. I tried this :

UPDATE tablename
IF columnA = '../../..' THEN
SET columnA = '0000-00-00' ELSE
SET columnA = STR_TO_DATE(columnA,'%m/%d/%Y);

Could use any help on the matter.

if-statement