Note that below is a guide to deleting a regular 1:1 or group conversation. If your conversation includes teams, navigate to this article instead: How to delete a team conversation?
To delete a conversation, launch the conversation actions menu (right-click on the conversation in the desktop version) Delete from the drop-down menu that appears.
Please note that once you’ve deleted the conversation you will have no access to this conversation or its content again. Other team members will not see that you’ve deleted the conversation and can continue the conversation. If your intent was to leave the conversation, use the slash command /leave instead.
If other chat members continue chatting in that conversation after you've deleted it, the conversation will re-appear in your conversations list (without the chat history).