This article provides an oracle important documents list for any versions which will help you to upgrade your database to any version.
Upgrade Oracle Database to 12.1:
Complete Checklist for Manual Upgrades to Oracle Database 12c Release 1 (12.1) (Doc ID 1503653.1)
Upgrade Oracle Database to 12.2:
Complete Checklist for Manual Upgrades to Non-CDB Oracle Database 12c Release 2 (12.2) (Doc ID 2173141.1)
Patches to apply before upgrading Oracle GI and DB to 12.2.0.1 (Doc ID 2180188.1)
Upgrade Oracle Database to 18c:
Complete Checklist for Manual Upgrades to Non-CDB Oracle Database 18c (Doc ID 2418045.1)
Patches to apply before upgrading Oracle GI and DB to 18c or downgrading to previous release (Doc ID 2414935.1)
Upgrade Oracle Database to 19c:
Oracle 19c – Complete Checklist for Manual Upgrades to Non-CDB Oracle Database 19c (Doc ID 2539778.1)
Patches to apply before upgrading Oracle GI and DB to 19c or downgrading to previous release (Doc ID 2539751.1)
