While holding the assembly with the speaker resting on the table, remove the four black Phillips screws holding the speaker in place and their associated rubber bumpers.

When replacing the speaker, place the rubber bumpers into the plastic casing first, then slide the screws into them, and finally line up the speaker with the screws. When working on the lower screws, you'll need to angle the end of the screw up a bit, as the speaker sits higher on the framework than you might expect.