This release brings more than 50 new features and over 70 bug fixes compared to the previous stable release (3.2.2). See https://github.com/Duet3D/RepRapFirmware/wiki/Changelog-RRF-3.x#reprapfirmware-33 for the change list and upgrade notes.
Users of Duet in standalone mode can upgrade from an earlier 3.x series release by uploading file Duet2and3Firmware-3.3.zip.
Users of Duet with attached Single Board Computer (SBC) should upgrade to this release from the stable package server.