Unless otherwise noted in the release notes, upgrading to a new version of Nexus is simple:

1. Download the new version of Nexus from the Nexus download page 
2. Unzip the compressed binary and copy the following folders to the server (overwriting the existing ones):
3. 1. /api
   2. /include
   3. /plugins (rarely changed but still you might as well)
   4. /themes
   5. <all the files in the root>

That should do it.  In cases where database changes have been made (major updates only), instructions will be included in the release notes. 

Versioning

Nexus follows a release early, release often methodology.  That is to say, you will find that there are many minor releases with one or two bug fixes each.  There are a few reasons for this that we won't get into here.  Visit the Nexus download page for a complete description of what each minor release contains - you can decide whether or not you want to upgrade based on that information.

Versions have four parts: major, minor, bugfix and build (1(major).7(minor).0(bugfix).5(build)).  The major   is only changed when a significant improvement has been made to the package.  Minor version field is updated on a discretionary basis but is usually reserved for a large set of fixes and changes that do not warrant a major point upgrade.  Bugfix and build increases more frequently and include incremental improvements and bugfixes.  It should be noted that with each new version the build number will change but the other three may not.  The build number is based on the Subversion changelist and automatically generated as part of the build process.