The 139.53681B is the same as most other Craftsman remotes, it just takes 2 batteries rather than 1.
Look for raised lettering on the back of the remote that says "Open this end first" and then insert the end of the visor clip or a small screwdriver into the seam between the front and rear covers at that point. Be sure to start at this point or you could break the plastic tabs inside the cover and your remote will no longer go back together properly.
Once you twist the clip, or screwdriver, the front and rear halves of the cover will pop apart. Pay attention to which way the battery, or in this case batteries, go in so you can be sure to get the new ones in correctly and the remote will work once it is back together. Typically the batteries go in so that the positive side faces down.
The remote goes back together opposite how it came apart, on 3 function remotes insert the tabs first then snap shut, on compact remotes snap shut along both sides. You can pull up a manual at Manage My Home with illustrations and more information using the model number "13953681" and the instructions in the link below.