I can't update the Gridsby theme

In order to solve my ‘older posts’ problem, I need to update to the latest version of gridsby. But every time I try to update the theme, it fails and I get: Could not create directory. It’s the same with every update I try to do. I’m not good with HTML, so I hope that somebody can give me a solution that is easy to do.

Thanks in advance for helping me out!