How do you change the battery in the keyless entry key?
I do not know what your keyless entry key looks like, but can give you some suggestions. Usually on the back of the key there will be tiny screws. You have to unscrew these and that is where the replacement battery will go. It will probably be a battery that is similar to a watch battery. Your manual should give this information if the keyless entry came with the car. The only other way I can think of is to pop apart one side of the key. I would check for screws first.
Search Volkswagen Passat Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
Volkswagen Passat Experts