Insert the SIM tool into the tray hole and gently push to pop it out.

To use one SIM card:

Or, to use two physical SIM cards (not available on all models):

Push the tray back into the phone.

If you inserted two SIMs, you'll receive a notification to set them up. Swipe the notification down and tap it to manage dual SIMs. If you see 
If you’re having problems, try these troubleshooting steps.