Can’t customise with theme

I’m working on a company’s website and something on there is so out of date, you can’t upload images in the normal way – just thumbnails, you can’t edit a page in ‘visual’ it has to be ‘text’ and I can’t seem to make any design changes. I’ve updated all plugins and themes etc. The site is using a child theme from twenty thirteen, seems to be bespoke but whenever I try to ‘customise’ it’s just a blank page with a small menu in the top left saying ‘Your are customising’ then blank!! Please help, how can I update this website design-wise and it musten get out.

It is an older website but now basic updates possible are left and it’s time for a new one.