To change your computer from mirroring to a secondary display, you simply need to adjust your display settings. Follow these steps based on the operating system you are using:

Windows:
1. Right-click on your desktop and select "Display settings."
2. Under the "Multiple displays" section, you'll see a drop-down menu labeled "Multiple displays." Click on it to expand.
3. Select "Extend desktop" from the drop-down menu. This will set your computer to use the secondary display as an extension of your main screen, rather than mirroring it.
4. Click "Apply" to save the changes, and then click "OK" to close the settings window.
Your computer should now be using the secondary display as an extended desktop.

macOS:
1. Click on the Apple menu in the top-left corner of your screen, and select "System Preferences."
2. In the System Preferences window, click on "Displays."
3. In the Displays settings, you should see two tabs labeled "Display" and "Arrangement." Click on the "Arrangement" tab.
4. On the Arrangement tab, you'll see a checkbox labeled "Mirror Displays." Uncheck this box to disable mirroring.
5. If necessary, you can drag and position the secondary display arrangement to your preferred setup.
6. Close the System Preferences window.

Your computer should now be configured to use the secondary display as an extended desktop.
Remember that the steps may slightly vary depending on your specific computer model and operating system version, but the general concept remains the same.