Update when third character is "_" - mysql

Question

I want to update column 1 in table A in a specific way: When the third character in the column is an "_" I want to insert the first 2characters, if the third character anythin else I want to leave it as it is.
Example:
|col1|
+--------+
|161_512 |
|16_1217 |
|161_512 |
|161512 |
|17_0117 |
|1615_12 |
Expected outcome:
|col1|
+--------+
|161_512 |
|16 |
|161_512 |
|161512 |
|17 |
|1615_12 |
Thats what I´ve got so far - but this is not working correctly:
UPDATE table A SET col1 = CASE WHEN col1 LIKE '%_%' THEN ... ELSE col1;

Answer 1

If you want to match a literal underscore in your LIKE expression, you will need to escape it using backslash. An unescaped underscore means match any single character. However, I would reword your query such that it uses a WHERE clause to determine whether or not to update a given record.
UPDATE table A
SET col1 = ...
WHERE col1 LIKE '__\_%'
Note carefully here that LIKE __\_% says to match any two characters, followed by a literal underscore, followed by anything else.
If you don't feel comfortable dealing with all this, you can always use a substring to check the value of the third character:
UPDATE table A
SET col1 = ...
WHERE SUBSTRING(col1, 3, 1) = '_'

Answer 2

When the third character in the column is an "_" I want to insert the first 2characters
I don't know what this means. First 2 characters from which attribute? Where do you want to insert them?
When the third character in the column is an "_"
That's simple. As you may already know, the underscore character is a wildcard matching exactly one character. Hence
LIKE '%_%'
will match any attribute with a at least one character. If you explicitly want to match an underscore character then you would escape the underscore
LIKE '%\_%'
And to require this to be the third character, perform a match on 2 single characters before it:
LIKE '__\_%'
(L I K E ' _ _ \ _ ')