After the post is sent, you have a limited amount of time to make any corrections. Not sure what the time is, maybe about 20 mins.
When you spot the error, at the bottom right of the post are normally three icons. After a new post, there is a fourth icon called edit. Press this and your text will appear in an edit window. In this you can place the cursor, click, back space, delete and change the text to your requirements. Once happy, just send again (save). This process can be repeated many times.
Not sure, but once someone replies to your post, this facility goes away. I use this feature all the time, as I am a bit picky about typo's. If I am writing anything longer than a paragraph or two, I write the text in a word document and Copy/paste into the TU quick reply box. Word has a good spell checker I like to use.