Did you watch the video Tester posted?
This video shows how to remove the door panel.
Do you plan to do a lot of work on your own cars in the future?
If not, it might be worthwhile to have a shop do the work. A new lock cylinder comes with new keys, new door key will no longer match trunk and ignition, just keep that in mind.
You continue to refer to your fob in the singular, I urge you to get a second fob and have it programmed to your car, likewise if you only have one ignition key (I think the anti theft chip is in the key) have a second key made and programmed, any locksmith can do that.
As far as your current fob, battery replacement is usually easy, but for convenience I go to Batteries Plus.