Hi, I had this same problem, thread here: Why isn't my Envato/themeforest theme letting me update? ERROR: The package could not be installed. PCLZIP_ERR_BAD_FORMAT (-10) : Unable to find End of Central Dir Records
copy and paste of my solution:
I solved it, though I’m not happy with what I had to do.
I downloaded the zip file for the updated theme that was emailed to me, and tried manually installing that. It said that it couldn’t because it already had a directory there. However, that gave me the location of the theme on my server: /var/www/html/wp-content/themes/soledad.
I went to my server and moved this folder (since I was afraid it wouldn’t save changes/options I had done for the theme) by going to that directory and doing:
mv soledad backup_soledad
(so it’s just changing the name of the folder and I could change it back if it broke stuff).
Then I updated from the dashboard and it worked successfully. Good luck. Lord knows why we have to do this.