These steps are for the web app.
Search by title
Click Search chats in the sidebar and start typing. The list shows only the chats whose title contains what you typed. Upper and lower case do not matter.
Search message text
While you type, a section called In messages appears under the list. It shows the chats that contain the same words in one of their messages.
Message search matches the exact wording you type. It does not match by meaning, so a synonym or a different word order will not find a message. Try the words that appear in the message itself.
If message search is not available at the moment, Bluedeer says that chats are matched by title only.
To rename, pin or delete a chat from the list, see How do I rename, pin or delete a chat?