If your discovery board is empty or you have very old firmware (below 1.4.0) - just copy-paste initial_firmware_v1.4.6.bin file to your board when it's connected over miniUSB.
If you have firmware v1.4.0 or later you need to copy specter_upgrade_v1.4.6.bin to the SD card and insert it into the device. The bootloader will check the signatures of the upgrade file and update firmware.
IMPORTANT: If you want to use self-signed bootloader and firmware check out the instructions in the bootloader repo
sha256.signed.txt file contains sha256 hashes of the firmware binary files and signed with @stepansnigirev's GPG key.
You can get the public key here: https://stepansnigirev.com/ss-specter-release.asc.
It is also available via keys.gnupg.net or keys.openpgp.org.
Fingerprint of the key is 6F16 E354 F833 93D6 E52E C25F 36ED 357A B24B 915F, short id: 36ed357ab24b915f