Important: Before upgrading, back up "sonatype-work/nexus/conf". If you need to downgrade for some reason, shut down the new server, restore the "conf" directory, and start up the old Nexus.
Since Nexus separates its configuration and data storage from the application, it is easy to upgrade an existing Nexus installation.
To upgrade Nexus, unpack the Nexus archive in the directory which contains the existing Nexus installation. Once the archive is unpacked, the new Nexus application directory should be a sibling to your existing sonatype-work directory. If you have defined a symbolic link for the version of Nexus to use, change that to point at the new Nexus application directory.
When you start the new instance of Nexus it will read the existing repository configuration from the sonatype-work directory and perform any configuration upgrades needed.
Also be sure to check the upgrade notes for version specific issues which may need to be taken into consideration before upgrading.