The similar problem when I try to update theme and core, the error below:
An error occurred while updating XStore Core: The package could not be installed. PCLZIP_ERR_BAD_FORMAT (-10) : Unable to find End of Central Dir Record signature
1) You are trying to update theme on the different domain that theme was activated (for example you activated the theme on localhost and moved it to live domain). Then you just need to go to Theme Options > deactivate theme and activate it using your purchase code again
2) There is no enough space on your server and archive that was downloaded for the update is broken. Make sure that you have no less than 20MB of the free space on your hosting.