To install the new diode:

Apply flux to the pads and add a thin layer of solder. If solder does not adhere to the pads, you can add it to the underside of the diode pins instead.

Hold the new diode in place using tweezers. While applying downward force, apply heat to one pin near the solder until the solder melts and makes a connection. Repeat with the other lead.

Verify that the connections are good. There should be continuity between one pin of D6 and a pin in VR3, and between the other pin of D6 and D7.