FlashBack 12c-19c failed upgrade.

If Upgrade fails 
In general, there are very less chance to fail the upgrade, If you complete precheck properly. Even so it the upgrade fails you can revert to previous version.

1. Shutdown immediate;

2. Set ORACLE_HOME to 12c or whatever you have the previous version.

3. Start up mount (with the 11g spfile)

4. select * from v$restore_point;

5. flashback database to restore point pre_upgrade; ( this restore point was created before upgrade)

6. alter database open resetlogs;