How do you replace keyless remote battery on BMW x5?

you do not ever have to replace the battery because there is no servicable battery to replace, there is a capacitor inside the key and that is used for the buttons to unlock and lock the. the way the capacitor charges is when its in the igntion and the key is turned on a antenna ring charges the capacitor. if the key does not work you may have to drive the vehicle a while, about 30 mins to charge the capacitor, or you may have to reinitalize it ( see dealer), or the key itself may just be defective and have to replaced.

The X-5 (and other BMWs) could easily have the Comfort Access option where you can leave the key in your pocket or bag and open/close and start the car without putting the fob in the ignition. These "keys" DO NOT charge in the ignition, they contain finite life batteries that need to be changed when they die. Easy process that's noted in the manual.