Customer's question:

I am trying to update my WordPress theme using the Upload theme button at the WordPress dashboard in Appearance -> Themes. I upload the zip file and I get the following message:

Unzipping ...
Installing the theme ...
The folder already exists. / home / .../ public_html / wp-content / themes / pe-theme-name /
The theme installation has failed.

Solution:

WordPress can't override theme files using WordPress themes upload functionality. I mean, this issue relates to all WordPress themes, not only PixelEmu themes.

You can remove or override the old theme folder and then replace it with the new one using FTP/SFTP. 
Please follow this tutorial How to Update Theme.