Hi Marina. I found this info in the [link|http://usermanualworld.com/wp-content/uploads/Samsung%20Galaxy%20R%20GT-I9103%20user%20manual%20guide.pdf|Samsung Galaxy R User Manual|new_window=true]:
To remove the SIM
1) If the device is on, press and hold the Power key and select Power off → OK to turn it off.
2) Remove the back cover.
3 Remove the SIM or USIM card with the gold-coloured contacts facing down.
Hope it helps.