Legitimate sources are rare. Avoid "driver download" websites that require a credit card.
If the previous technician or a virus corrupted the boot block, the board will not even attempt to read the SPI flash. A "verified" file has its checksum (MD5 or SHA-256) validated against known-good working copies.
: Many BIOS files come with checksums (MD5, SHA-1, etc.) to verify the integrity of the downloaded file. You can use tools like md5sum on Linux or macOS to verify the checksum.
To ensure compatibility, verify that your hardware matches these specifications:
Are you planning to yourself with a hardware programmer, or are you just looking for a software update ? USB BIOS version 5.20 for Satellite C40-C (PSCRJx)