Backup Your Database
The upgrade process changes your database. It adds tables and indices, and amends the tables that are already there in some cases.
These are not changes that your database server can roll back from on its own. If something goes wrong during the upgrade, for instance one of the datafiles runs out of space, or your machine suddenly looses power and is switched off, you'll have no option but to restore from a backup.
Make sure you have a backup to recover from.
Consult your database vendor's software documentation for information on how to make and verify a backup.
There is also a useful link about backups here.