You must design and carry out an appropriate backup strategy to ensure a successful upgrade.
To develop a backup strategy, consider the following questions:
How long can the production database remain inoperable before business consequences become intolerable?
What backup strategy is necessary to meet your availability requirements?
Are backups archived in a safe, offsite location?
How quickly can backups be restored (including backups in offsite storage)?
Have recovery procedures been tested successfully?
Your backup strategy should answer all of these questions and include procedures for successfully backing up and recovering your database.
Oracle Database Backup and Recovery User's Guide for information about implementing backup strategies using RMAN