Hi Karen, you've not said which model of remote keypad you have, but in general, most keypads will need to be released from the bottom edge and swung upwards to allow you to unclip the top edge from it's fixing tabs.
To release the bottom edge you will either have two little screws to take out or loosen on that bottom edge or there will be two slots that you insert a flat screwdriver into and prise open to unlatch the hidden fixing tags. Remember that your alarm will start sounding as you open this keypad as it thinks it is being tampered with, so be ready to enter the code on the main box. Once this keypad is removed, you will find the wall fixing screws on the backplate.
Rupert, March 2020