can’t update theme

hi – I have a couple of wordpress sites – I’ve been updating my business one at http://www.storyflex.tv

It’s running a very old 2011 theme at the moment so I’ve tried to update it – and when I ‘preview’ it – it just shows me the same theme I’m currently using, and then when I activate the new theme the site vanishes completely.

I thought maybe it was because it was an older version of wordpress the new theme wasn’t combatible – but I’ve recently updated to the latest version and am still getting the same issue. any help gratefully received