CASE WHEN wrong syntax

Databases
1

Hi guys and good weekend for all.

I can not working this query update with CASE WHEN… What is wrong in this syntax?
I hope your appreciated help.
Saludos!

UPDATE tbl_upd t
JOIN(
	SELECT
		(
			FIRST SELECT
				...) + (LAST SELECT ...) output1)output2
			SET CASE
			WHEN LENGTH(strVariables)= 2 THEN
				t.FieldUpdate1 = output2.output1
			WHERE
				xCodes = CONCAT(strVariables, '12')
			ELSE
				t.FieldUpdate2 = output2.output1
			WHERE
				xCodes = strVariables
			END;





[Err] 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 'CASE WHEN 
          LENGTH(strVariables) = 2 THEN t.FieldUpdate1 = output2.output1 WHERE ' at line 41
2

you can only SET a column, you cannot SET a CASE expression

3

thank you!