This release fixes bugs in RC3 and brings some minor feature improvements. See https://github.com/Duet3D/RepRapFirmware/wiki/Changelog-RRF-3.x-RC#reprapfirmware-350-rc4-changes-since-rc3 for the changes since RC3.
Users of Duet with attached Single Board Computer should upgrade using the unstable release from the package server.
Standalone users should upgrade using the files here. You can upload file Duet2and3Firmware-3.5.0-rc.4.zip to your system (which will also update the WiFi firmware to version 2.1beta7) to upgrade all relevant files automatically. Alternatively, you can upload the individual files needed by your system. There is no need to update your WiFi module firmware if you are already running version 2.1beta7.
Note, if you do update the WiFi firmware on a Duet 2 or a Duet 3 Mini and you were previously running version 1.27 then you may need to re-enter your access point details using M587.