From what it looks like a WhastApp certificate is issued from your mobile phone's software and can not be downloaded. When given an error that states that your certificate is expired this either means:
1. The date, time and/or year in your device are incorrect.
2. You need to update your device's software to the latest available version.
WhatsApp seems to be phasing out "old" devices quicker than they had initially advised (http://www.techzim.co.zw/2016/02/estimated-10-zimbabwean-smartphone-users-affected-whatsapp-ends-support-blackberry-nokia/
Best bet is to get a new phone (I'd recommend Android v4.1 upwards)