The default App clients will see in their sidebar is the Messages App, but you can rearrange the order for your clients. Note that the order will not change in Copilot Dashboard, only in the client experience.
To rearrange the order of Apps, simply go to App Setup in your sidebar and drag and drop Apps into the desired order:
Be sure to click Update in the top-right to save your changes. Once you've updated, go to the client experience to preview your changes.