Open File > Preferences... to reach the settings dialog. It has a sidebar with five pages, each covering one area of the program. Defaults are sensible, so you can leave most of it alone and change only what you need.

The pages are General (how computer status is checked, the default timeout, and how many machines are messaged at once), Computer list (how the list looks and refreshes), Appearance (light and dark themes), Logging (the application log and optional file logging), and Updates (how LanSend checks for and installs new versions).