Database Upgrade to 12c Steps Summary [ Definition only]

Upgrading database to 12c steps
=========================
Please read through below metalink documents for upgrading database to 12c:

MOS    #1503653.1 + Upgrade Companion #1462240 + upgrade script 3884522.1
Note 2037154.1 DBMS_STATS.GATHER_DICTIONARY_STATS Fails with "ORA-06502: PL/SQL: numeric or value error: character string buffer too small"
hcheck.sql - Script to Check for Known Problems in Oracle8i, Oracle9i, Oracle10g, Oracle 11g and Oracle 12c (Doc ID 136697.1)       May 2016
Script to Collect DB Upgrade/Migrate Diagnostic Information (dbupgdiag.sql) (Doc ID 556610.1)

In our workplace we are using a single scripts which contains steps below  -- verify into developemnt setup before implementing into working (TEST/QA/PROD)

PreUpgrade Phase:
                1  : Compile invalid objects
                2  : Create script to disable user triggers
                3  : Disable concurrent statistics gathering
                4  : Fix invalid user table data
                5  : Execute Database Health Check script
                6  : Run the new Pre-Upgrade Information Tool
                7  : Check for the integrity of the source database
                8  : Purge the Recyclebin
                9  : Gather dictionary statistics
                10 : Select and save current invalid objects

Upgrade Phase: [ This is based on Dataguard and RAC setup with OEM monitoring in place]
                1  : Create blackout on target
                2  : Disable transport and apply when DG available
                3  : Determine Database Type
                4  : Disable database parameter cluster_database
                5  : Compile invalid objects before upgrade
                6  : Select and save invalid objects before upgrade
                7  : Disable and Stop database with srvctl
                8  : Rotate alert.log before uprade to compare after
                9  : Move database related files to new ORACLE_HOME
                10 : Modify oratab
                11 : Start database in upgrade mode
                12 : Create guaranteed restore point for rollback
                13 : Disable triggers
                14 : Determine and resize fast recovery area if necassary
                15 : Execute catupgrd.sql script to upgrade the database to 12c
                16 : Start database in normal mode again
                17 : Remove deprecated parameters
                18 : Compile invalid objects after upgrade
                19 : Execute post upgrade script utlu121s.sql
                20 : Execute post upgrade script catuppst.sql
                21 : Execute post upgrade diagnostic script dbupgdiag.sql
                22 : Modify CRS database configuration with new ORACLE_HOME
                23 : Modify database parameters after upgrade
                24 : Enable and Start upgraded database from new ORACLE_HOME with srvctl
                25 : Select and save invalid objects after upgrade
                26 : Enable triggers
                27 : Validate APEX installation after upgrade
                28 : Modify EM12c ORACLE_HOME target property
                29 : Stop blackout on target
                30 : Delete blackout on target
                31 : Rotate alert.log after upgrade for comparison
                32 : Create a full backup after the upgrade
                33 : Show database components
                34 : Check invalid objects before and after the upgrade

Then Upgrade Standby:
                1  : Determine Database Type
                2  : Determine standby database open mode
                3  : Remove deprecated parameters
                4  : Disable and Stop database with srvctl
                5  : Move database related files to new ORACLE_HOME
                6  : Modify oratab
                7  : Modify CRS database configuration with new ORACLE_HOME
                8  : Enable and Start upgraded database from new ORACLE_HOME with srvctl
                9  : Stop blackout on target
                10 : Delete blackout on target