According to the download page it states:
For BIOS 50 Note: Before updating BIOS to this new version, you MUST follow the steps as detailed under BIOS F40 description
For BIOS 40 Note:
1. If you are using Q-Flash Utility to update BIOS, make sure you have updated BIOS to F32 before F40
2. Before update BIOS to F40, you have to install EC FW Update Tool (B19.0517.1 or later version) to avoid 4 DIMM DDR incompatibility on 3rd Gen AMD Ryzen™ CPU.
From this, I believe that as long as you update the Firmware with the EC FW Tool first, you can then go straight to BIOS version 51.
That said, why do you want to update to BIOS version 51?
Is it because you need support for a new CPU or are experiencing issues?
If it is just because it is the latest version, but your motherboard and PC are running fine then, there is really no need to update the BIOS at all.