The TEI framework that is bundled in Oxygen XML Author Eclipse plugin has the TEI schema and stylesheets that were available at the time of its release. When the TEI Consortium releases a new TEI version (Schema and Stylesheets), they also release a new version of the Oxygen XML Author Eclipse plugin TEI framework with them.
The following procedure describes how to install a release of the TEI framework done by TEI Consortium that has a newer version of the TEI Schema and TEI XSL https://github.com/TEIC/oxygen-tei/blob/master/oxygen-tei-plugin.md