Whatsapp for blackberry 8700 os 4.5?

Whatsapp was working perfectly up until a month ago. I uninstalled it and when I tried to install it again I got an error saying "sorry your current blackberry os is too old and not supported. Please upgrade your blackberry os to version 4.6 or higher"

can someone please help. This is really urgent andimportant to me. I guess the only way to get around this is to either upgrade blackberry software or somehow find an old version of whatsapp. I don't know how to do either. Please help me