1. Release the paper-clamp latches. There will be one or two latches on both sides of the sander. For that model, they probably look like this (note the 2 red arrows, showing the release motion):

   

2. Pull the folded-over edges, of the sandpaper, out from under the clamps.

3. That model may use velcro paper, so expect that you might have to peel the sandpaper off of the base plate.

4. Replace with the same style and size (length/width) of paper. Assemble in reverse order. Pre-creasing the folded edges works best for getting the edge under the clamps.

You can find manuals at http://www.makita.com/en-us/Modules/Service/ServiceCenters.aspx.
Here is a manual for a similar palm sander (pdf) (AKA "Finishing Sander").