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

Insert or remove the cards as needed.
To use one SIM card:

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

To see if your phone supports two physical SIM cards, go to Settings > About phone > SIM status. If you see SIM Slot 1 and SIM Slot 2, then you can use two physical SIMs in your phone.
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.