Here are the steps to install a BIOS update on a MSI B450 Tomahawk:

1. Download the latest BIOS update for your motherboard from the MSI website.
2. Create a bootable USB drive using the downloaded BIOS update file and a tool such as Rufus.
3. Turn off your computer and disconnect all peripherals.
4. Insert the bootable USB drive into a USB port on your computer.
5. Turn on the computer and press the key to enter the BIOS setup, usually “Delete” or “F2”.
6. Navigate to the “M-FLASH” or “BIOS FLASHBACK” option in the BIOS setup.
7. Follow the on-screen instructions to select the bootable USB drive and start the BIOS update process.
8. Do not turn off the computer or interrupt the update process. The update may take several minutes to complete.
9. After the update is complete, the computer will restart automatically.
10. Enter the BIOS setup again and check that the BIOS version has been updated.
11. Save the changes in the BIOS setup and exit.
12. Turn off the computer and disconnect the bootable USB drive.
13. Turn on the computer and check that the system is functioning normally.

Note: The steps and the options may vary depending on the version of the BIOS. Please refer to the motherboard manual or MSI support for further information.