Ok, when you start a topic.
Then save it.
You can modify it.
When you lock the topic.
Then you unlock the topic to make some alterations.
There is no way to modify the post.
Strange.
- There's a 1-hour grace period for editing.
- Locking and unlocking simply means: Allowing / Not allowing new posts to be made in the topic.
Hope it helps.
Regards.