While the time limits may change and vary slightly, for a certain period of time after posting you can edit your own posts without any interaction from the moderators.
Simply click on the EDIT button in the top right corner of your post, edit away, then click submit. Helpful for a short period of time after posting and if you double post there is even a delete button available for a short time.
Once this time has expired, a little more effort will be required to modify an existing post.
Why can't I just edit my own posts after the time period? There have been problems in the past where angry users or hacked accounts would go and delete their old posts or spammers write innocent posts only to be changed to spam at a later point. It's a bit of a balancing act, you want the edit time long enough that you can make a quick edit and not so long that the spammers take advantage of it after it's off of the moderation teams radar. It makes sense to really take your time and think about your posts before you post them.
This is more useful for long term posts, like stickies or build threads.
The steps are as follows:
1. Copy the text from your original post
2. Create a new post in the Testing Grounds
3. Paste the original text into the new post
4. Edit away until have it exactly how you want it. if you are adding new pictures, please use the Gallery function to expedite the process. Then click submit.
5. Report your original post, giving the link to the new post in the Testing Grounds so that a moderator can copy the new post over to the original post. Reporting is not something negative, it just says 'hey this needs some attention'.