How do I modify a message already created in the forum?

I want to modify the content of a message created by me, and I can't do it
it's possible? and if so, how?

You have to click on edit icon. If you do not see the pencil icon , most probably it is because you are
TL1 - Basic user in the community. From TL2 - Member and above users can edit their own posts for up to 30 days after posting