Download and install Line 6 Monkey.
Connect your HD300 to the computer via usb, run Line 6 Monkey, and follow its intructions.
Begin by installing the HD300 Drivers. Pay careful attention to Line 6 Monkey's instructions about when to disconnect and reconnect your HD300.
Next, use Lin 6 Monkey to guide you through the download and installation of the most recent HD300 firmware and HD300 Edit.
You will find more help, including access to the knowledge base and instructional videos, in the Support section of this website (see tab at top right of this page).