Needed:

Wrench 11mm
Hex key 3
Lint free paper
Isopropyl alcohol/ethanol/methanol/acetone if necessary

Turn the power off and unplug the power cable.
Open up the ND25 by removing the screws and taking the cover off.

Be careful, the cover is attached to the unit via a cable that powers the cover's LED lights. Detach this cable before attempting to fully remove the cover.

3. Take all the connectors off the mainboard. Make sure you have a picture of how the cables were attached so you can replicate this when reinstalling the mainboard.

Unscrew the plate that holds the mainboard and take this off.

Release the metal tubing from the PMT. The top one does not come out, don't force this.

Release the 4 screws from the front cap of the PMT. Slowly open the PMT, the glass well.

Take the glass out. check for damages and clean the glass if needed.

nol.

Without touching the glass, place it back in the PMT. When placing the cap back don't forget to slide the metal tubing back into the too.

Place the screws back in the cap and screw them back into the PMT.
Place the rest of the metal tubing back and tighten the couplings. Don't tighten them too much or you'll damage the couplings.
Place the main board back and screw the 4 screws back in place.
Reattach the couplings.
Reattach the coupling to the cover LED's and place the cover back. place the 4 cover screws back.
Plug the power cable back in and turn the unit back on.

Cleaning the protection glass