Switch the version in System / Update / System Update from the new version to the current version
Under Diagnostics / Command Prompt, run the following command:
pkg-static clean -ay; pkg-static install -fy pkg pfSense-repo pfSense-upgrade
Back under System / Update / System Update, switch to the new version.
Go back to Diagnostics / Command Prompt and re-run:
pkg-static clean -ay; pkg-static install -fy pkg pfSense-repo pfSense-upgrade
Now, you should be able to pull down the latest version and install the update.