...Go in on 'Edit Text', click on the box near the top of the post. And scroll down on the page you get until you see the window that looks like the one you originally typed.
...Change your typing in any way you wish...'Backspacing' over parts you want to erase, or 'highlighting' that type by dragging your cursor over it until it is surrounded with dark blue, then hitting the 'Delete' button.
...Change the title of your post in the same way.
If you will be erasing the whole post, most folks like to write 'n/t', for 'no text', in the body and in the title, so people don't waste time looking at it.
Also, you could write something different, if you want to say something different. When I do that, I like to say something in the title that will call people's attention to the fact that the text has been changed.
'[Updated opinion]' would do, or any other that pleases you.
Deleting all or many of ones posts could be done, one at a time, in the same way. I did see the Webmaster delete all of at least one person's posts, a while back, on request, but I don't know if that service is still available, or even where to ask for it.