You will need to go into your GTA folder and into scripts; you will find your firstresponse.ini file, you will need to edit that so that where it says:
Hardcore = False
Then you will obviously want to save the file.
I do not believe there is anyway to toggle this while in game. Basically if I am doing missions I will turn hardcore off. If I want to do LCPD I turn it on and that is all that I do.