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 whose messages match what you typed. Message search needs at least two characters.
Message search finds the words you type, in any upper or lower case. It also finds messages that say the same thing in different words, when that check is available. If it is not available, only the exact wording is checked, and Bluedeer says so under the results.
If message search is not available at all, 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?