With the clock on its face and if possible, holding the case to ensure that the vertical pillars hold the glass panels, remove all four screws in the base and remove the base.

Check to see whether any of the glass panels is now loose and at risk of falling out. If so, carefully remove it and put it aside. Put the case aside standing upright, so that at worst, the glass panels can only slip down in their grooves until they are resting on the table.

On reassembly, if any of the glass panels and their plastic strips came out, ensure that the strips are at the far ends of the grooves from the base otherwise you may not be able to get all the screws in the base to engage with the threaded holes in the pillars.