Suppose I found an answer from a few years ago, a Python script or shell script, maybe C code. It works and answers the question, but it's buggy, the approach used isn't optimal, or messes up user configurations, and I suspect I could probably fix it. What are the options in such case ?
The question is a bit unclear on the specific situation, so it depends.
Deleting an answer is only an option if it is a really bad answer, or even does not attempt to answer the question.
Jacob answered what you should do, and you've given a good answer for what you should not do:
vote to delete that answer and post the edited version of the answer
I'd be incandescent if out of the blue somebody tried to take my post and delete the original. There'd be meta posts. There'd be moderator messages. I'd be a complete diva about it.
Hell, I'd be disappointed if people just tried to delete my posts without giving me some sort of notification to let me know there was a problem. There is no expectation that everybody will get everything right all the time, but at least give them an opportunity to fix it, if you won't do that for them.
You could also argue deleting the original is self-defeating. The CC licence here requires attribution to the post as well as the user. You delete it and you suddenly forfeit one of the conditions for your license. This argument is obviously not battle-tested but it's probably not worth getting in the situation where you have to make a legal argument for your right to use somebody else's content.
I would [and would expect others to naturally] follow Jacob's list there. Edit. Comment. If it's irredeemably awful, flag it (delete is too slow) but if it's that bad, you're not going to be reposting it.
