You push the button so that the key comes out. Then look inside where the key is usually you will see a little tab sticking out. You use a jewlers or other small flat head screw driver. You push in the tab and at the same time you pull the key out. Then you can separate both halves of the fob and replace the battery. You separate them with that same screwdriver prying them apart slowly.
Glad that i can help. If you have the owners manual it shows a picture of how it comes apart to replace the battery. I don't remember if the instructions are as detailed. Anyway glad that you were able to replace the battery.