Hello,
Thanks for the topic.
Please read our documentation https://www.8theme.com/documentation/xstore/theme-installation/theme-update/
Make sure you are trying to upload ZIP Compressed version or do the manual update.
Also, you can deactivate theme via Theme Options http://prntscr.com/ii7pyb . Then activate it again and try to update theme or plugin. Let me know about results
Regards