When upgrading within Dataverse 4.x, you will need to follow the upgrade instructions for each intermediate version.
Upgrades always involve deploying the latest war file but may also include running SQL scripts and updating the schema used by Solr.
Please consult the release notes associated with each release at https://github.com/IQSS/dataverse/releases for more information.
Upgrading from DVN 3.x is actually a migration due to the many changes. Migration scripts have been checked into the source tree but as of this writing it is expected that people will require assistance running them. Please reach out per the Introduction section.