You just click the "Edit Post" button, in the upper right part of the post.
I circled it in red for you in the attached picture.
It is worth noting, though, that while you can edit your own posts, if you want to change the title of a thread, you'll have to have a moderator do it for you, as we don't let users edit threads. You can use the "report to moderator" button (the little "!" in a triangle at the bottom of each post) to let them know that you need help.
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