LEFT and RIGHT are just functions that return a string; you can't SET the LEFT/RIGHT characters of a string like this (it would be like trying to do SET GETDATE()=...).

There are a variety of combinations of string functions you could use to satisfy this; I'll just show one possibility:

DECLARE @test-2 TABLE (some_string VARCHAR(MAX));

INSERT INTO @test-2 VALUES ('17test01'),('17something05'),('16morerandomstuff11');

SELECT modified_string=STUFF(LEFT(CONVERT(VARCHAR,GETDATE(),12),4),3,0,SUBSTRING(some_string,3,LEN(some_string)-4))
FROM @test-2;