To release a jammed handbrake, you should 

1. Use both your hands and lift up the handbrake fully.

2. Once lifted, press onto the button of the handbrake and push it down. 

It should be released.

Otherwise, you can seek help from a passerby as most probably the previous hirer had over extended the handbrake, causing it to be stuck.

Please note that this is an operational issue and not a mechanical issue.