To edit a message on the desktop version of Fleep, right-click on the message and choose Edit from the drop-down menu that appears. You can also find all other message actions from the menu - Quote, Copy message, Forward, Mark unread, Pin, Create task and Delete:
On the mobile apps, you can launch the message actions menu with a long tap (long press) on the message.
Note that you can Edit messages that you have posted, as well as any Tasks and Pinned messages.