Mac with 2 monitors. How to change screens?

Solution 1:

One thing you could try is resetting the NVRAM of your Mac to see if that resolves this.

You don't specify the model of your Mac, so below are some generic steps that will work to reset the NVRAM on all Mac models:

1. Shut down your machine. Yes, a full shut down, not just logging out.
2. Press the power button and then immediately press the commandoptionpr keys.
3. Keep these keys pressed down for at least 20 seconds!
4. Let go of the keys and let your Mac reboot normally.

Note: When you log back in you may need to readjust some of your system preferences (e.g. speaker volume, screen resolution, startup disk selection, time zone information, etc).

Solution 2:

Make sure you are plugging in the monitors in the same ports and in the same order, as the OS will add the displays as they appear.