Easy to do. First remove the plastic pin from the surround of the door opener. Then, starting at the bottom left corner of the door panel, pull it away from the door. The door card is held in place by plastic clips. There are quite a few of them, but if you work your way from left to right along the bottom you will be able to unclip them all and remove the door card.
to replace the bottom speakers, this IS all that it required. To remove to top speakers, then top part of the door card has to come of, but it is no more difficult to do. I know because I have just taken mine of.