You would have to do it in the database with something like phpMyAdmin. You need to change the time on the topic and the reply, and I think maybe there is a spot where it is reflected in the forum table too.
The “Edit Post Attributes” plugin lets you edit timestamps and even usernames!
https://bbpress.org/plugins/topic/edit-post-attributes/
I use it on several sites and haven’t had an issue with it yet… highly recommended.
thanks guys, that works great!
This page doesn’t exist. Does anyone know if this plugin still is available?
This functionality is now available in WordPress core. If you edit the post using the WordPress Admin area you can edited the Published date.

View post on imgur.com
(Your post editing screen may be arranged differently than mine)