Install the spindle belt  onto the spindle.
Hoist the replacement gearbox into the machine.
Install the (4) gearbox mounting screws .
Adjust the belt tension using the jack screws located at the bottom of the gearbox mounting plate. Refer to: Spindle Drive Belt - Tension Adjustment - Gates Sonic Meter - Mill
Torque the gearbox mounting bolts . Refer to: Haas Fastener Torque Specifications
Remove the lifting eye bolt.
Clean the gearbox oil reservoir  to remove debris.
Connect all gearbox cables and hoses to the replacement gearbox.
Reinstall the TRP, spindle fan, waycovers, and enclosure panels.