Use checklists to plan and carry out Oracle Database upgrades.
Oracle recommends that you use checklists as part of your upgrade planning and implementation process. Using checklists can help you to use available upgrade utilities, and can help you to ensure you carry out a successful upgrade.
(Look under $ORACLE_HOME if $ORACLE_BASE is not set). Also, folders with the system identifier (SID) of individual database are in this timestamp folder. The SID folders contain files for individual databases for the preupgrade and upgrade process.
Verify upgrade status for CATALOG and CATPROC components
select substr(comp_name, 1, 45) as comp_name, substr(version,1,8) as version , substr(status,1,8) as status from dba_registry
Look for version and status changes.
Run Oracle Database 12.2 Post-Upgrade Status Tool (utlu122s.sql)
Update time zone settings
If Time Zone version at source database is lower than the target database, then run Time Zone conversion after the upgrade.
Time Zone defaults are in the path $ORACLE_HOME/oracore/zoneinfo
Create Oracle Database system files
Create an SPFILE from the PFILE
Gather new Oracle Cost-Based Optimizer (CBO) statistics