As a new user you can edit your own posts for up to 24 hours, and that should include topic title. This is generally to prevent a new unknown user from posting an innocuous message and then either changing it to spam or abuse later, or a new user from covering their tracks if they misbehave (“What? I didn’t post that! Look!” - even though I believe post edits are visible to everyone.)
When you hit trust level 2 (by participating and reading other topics and posts) then you (currently) have the option to edit any of your own posts without a time limit, and Regulars at level 3 can update the title/topic/tags of any posts they can read so they can help with forum organization and making vague post titles clearer.
This is of course adjustable by Admin - if we get another run of users trying to mass-delete their content or erase their identity as some have done, we can always limit how long a post is editable for.
As always, your friendly neighborhood Forum Staff team can assist if you need to change or edit a post that you don’t have access to.