I am new to iphone 5s. I need to know how to remove the back to insert the sim

Actually you do not remove the back. Its on the side:

http://help.apple.com/iphone/8/#/iphcf42bbc36

You do not remove the back to insert the SIM. See this support document and locate your device and read how to open the SIM tray to insert your SIM. Remove the SIM card from your iPhone or iPad - Apple Support

You may like to read this to understand how to set up and use your iPhone

https://manuals.info.apple.com/MANUALS/1000/MA1565/en_US/iphone_user_guide.pdf

and it might be useful to understand that the iPhone is not user serviceable in any way

any attempt by the User or an unauthorised repairer to open the iPhone will void any support or help from Apple on the device

This obviously does not include the sim tray 😉