Since 1.10 has not been finished/released yet, I guess you're talking about one of the beta/rc versions. Please note that these not be used in productive environments!
But yes you can revert back to 1.9.12 when you have taken a backup prior the upgrade.
So all you have to do is to recover the MySQL database backup and revert the osTicket PHP files back to 1.9.12.
If you haven't done a backup, I think it would be really complicated to revert back things.