Can you download the theme package and unzip it manually? The most likely cause of the error is that the zip is corrupt or that it was not compressed with zip.
If you can unzip the theme package, you could then upload the latest version via FTP to the folder /wp-content/themes/mywriter (please note the actual folder would be case sensitive so if its myWriter or MyWriter or some other variation be careful to upload it to the right place).
Or you could try rezipping it (using zip) and upload it via WordPress admin.
Incidentally, if you are intending to customise the theme substantially, then you should use a child theme. Otherwise next time the theme is brought up to date by the theme author you could stand to lose all of your customisations. For more information about creating a child theme, the official WordPress guidance can be found here https://codex.wordpress.org/Child_Themes and is explained more simply here https://www.smashingmagazine.com/2016/01/create-customize-wordpress-child-theme/
Hope this helps.