The forums system gives you the ability to edit your posts. Here are instructions for how to edit for specific kinds of changes. Please read on down to what other kinds of changes not to edit.
Simple edits
Even though you can preview your posts, and even though the system underlines obvious spelling mistakes, everyone knows you can't see a typo until after you've submitted a posting! So it's easy to correct typos and hyperlinks that don't work. On your own postings, within 5 minutes for new users, or any time for everyone else, you can click the Edit button at the bottom of the text area, and you will be taken to a screen where you can enter and correct text. The toolbar is the same as the one where you entered your posting, and you can mouse over the icons to see what they do.
The best time to make typo corrections is immediately, and you could test your hyperlinks and correct them immediately. And if you find that your point isn't clear enough and you want to add some formatting, sure - go ahead and do it. Or if you copied in some text and it turned out to be teeny, you might find that just highlighting the text and clicking the "remove formatting" button fixes it right away.
More advanced edits
If you want to add or remove photos, or move them into the text area, on the screen after you click the Edit button, click the More Options ... button.
Here is a video showing how to do a basic edit, and how to delete and add photos.
What not to edit
OK. This ability to make changes, and the ability to delete your posts (again, for 90 days), is a hot topic. Let's say you posted something identifying some plant as a Qubitismo qubittus, but then someone said that it wasn't that at all, it was something else, and now you feel foolish. So you're tempted to go back and change what you said. The thing is, it was a conversation, and that person and others who replied took the time to contribute, and what they said makes sense in the context of your original posting. If you change your posting, or worse delete it, it can make the rest of the thread confusing or meaningless. So don't do that.
It is also considered poor form to alter a post such that a reader approaching the thread for the first time would be deceived into believing that the modified conversation represents an actual sequence of posts over time. It's better to click Reply to add or retract information, if the edits you are considering would alter the natural flow of conversation.
Another thing about editing posts is that people don't get notified of edits, but people who have posted to a thread, and other people who have subscribed to a thread, get notified when there are new postings. So if you add some information via editing, but people have already read your posting, they may not ever find out that you corrected or added to what you originally said. If you make your correction or addition as a Reply, then everyone can see that there's a new posting, so your new information will reach more people.
So why is the ability even there to make edits? Well, you might have got the photos in the wrong place. Also, it is desirable in some areas of the forum to be able to use a single post to gather information and make subsequent edits to add, remove or clarify information. I do that in the Help and Resources area, where there is only one posting in each thread, and I try to keep it as complete and current as I can. Or in the VCBF forums, where it seems important to get the id right, as the threads are less about conversations and more about conveying information about what trees to find where. If you're making corrections via edits, at least be sure to indicate what's been changed and why, and consider whether someone elsewhere has referenced something you're about to change.
Edit your posts
How to Edit your posts, and when is that appropriate