Hi,
first of all put some bits of tape on the retaining ring and onto the inside edge of the tube so you can put it back exactly the same way, then when you remove that ring, but a mark on the edge of the glass and also on the inside of the tube, so you can put the glass in exactly the same orientation, there may well me a mark already on the glass as this is usually done when built.
then there should be some small cork or maybe other material, packers around the glass, donβt loose these, and make sure you put them back evenly around the glass
then holding the secondary housing remove the glass, but do this with the tube horizontal or slightly pointing down, so no bits go inside the tube, clean with a brand new untouched micro fibre cloth with Baader wonder fluid, spray it on and then gently wipe over until the liquid has evaporated...
Then fit back by doing the reverse, it is very important to put all back in the same orientation as it was....
HTH ππ