If the back is removeable, that is with a load of screws around it, then take out the screws, take the back off and unbolt the speaker. No screws in the back? Then the front cloth is vecroed on. Gently prise it off, undo the speaker bolts, they'll be bolted into T nuts and remove the speaker but be careful of the wires to the speaker. Disconnect these before pulling the speaker right out.