The "unsend" feature will only work for up to 10 minutes after a message has initially been sent. Locate the message you want to "unsend," and long press on the text bubble. Along with the reaction emojis that appear, a row will pop up at the bottom of the screen with three options: "forward," "save," or "remove." Click "remove."
After you press "remove," you'll be given the two options for what to do with the message: "remove for everyone," which will delete the message for all parties in the chat, or "remove for you," which only deletes the message from your side of the chat — in case you want it removed from a screenshot, for instance.
The second option is available indefinitely for any message you want to delete, even if it's been longer than 10 minutes.