How can one edit/delete his/her post days after posting it?

You can't. Editing your post is allowed for 30 minutes. Deleting isn't allowed at all, unless the post violates one of our forum rules. Minor edits can be requested by using "Flag Bad Post" to request a moderator's attention.

editing posts can in some cases also misunderstandings, comparable to the diamond of death person:

let's say the OP asks a question that can be solved in two ways, say approach one and approach two.
a first person replies, explaining the first approach.
now, he posts his reply, and a second person knows something that might improve, or stabilize some issues in that approach, and he adds something like:
'but if you want to do it like that, you'll need to change x and y in your config like this ...'

at this point, the first person who replied comes back, because he decided that explaining the second approach is easier for someone who just started working with it, and edits his post, and replaces the entire explanation by an explanation for the second approach, without realizing the second persons addition.

what if at this point the OP comes back? he implements the second approach, and adjusts the configuration to improve the other.

same goes with deleting posts: every post has built to the sollution. why deleting it? it might come in handy to those who have a similar problem and are looking for the answer. a step-by-step tutorial is pretty useless if you start cutting out the steps, don't you think? :)