You can delete any message you send. In addition, an admin can delete any message sent by anyone in the group in a topic or event chat. Reactions on deleted messages are also removed.
When a message is deleted, no notifications are sent, and any text or content is replaced with “message deleted.” Note that the whole message is not removed because others may have already seen the message and get very confused as to why it’s not there anymore.
To delete a message, open the message menu by long pressing or tapping on the mobile app, or clicking on the desktop.