Land Rover Discovery I need to replace batteries in key fob, which way do batteries go in and will it need to be re-programmed

Has the battery ever been replaced? It may only need a new battery. When the battery has been replaced it will need to be 'synched' to the central locking. A few links to help you -
Key fob not working

How To Replace Land Rover Freelander Key Fob Battery 2002 2003 2004 2005

Yes it has a battery, two possibilities a rechargeable one, or a 2032. if you have the full black plastic cased fob it is a rechargeable battery. (most likely version)

to replace that will require some deft soldering skills. may writes ups available on multiple Land rover message boards

A dealer replacement is the only option to reset the system and that is an expensive fix. You can buy a replacement fob online but be careful - it will need to be fully compatible or you run the risk of locking yourself out OR losing the central locking function.
I have no connection with this seller, but at least they make all the right noises:
https://www.keylessentryremotefob.com/Land-Rover-Key-Fob-Remote-s/1836.htm
Do more homework - especially on the specialist Land Rover forums before buying

Easiest way is to find the manual on line, if you do not already have it, and pull the fuse for the alarm. That should disable it. Then next dealer visit you can have it reset and you'll be good to go.

this is actually the internal DC motor which comes inside the door latch. Land Rover doesn't sell it separately. You need to buy the whole door latch to have this problem sorted out. or else you can buy the DC motor from me as I made a special order from a factory for those DC motors as they are common fault in most of new Land rover generation. each motor costs you $30 +shipping Regards..

if its a replacemet fob it can only be done with diagnostic equipment
if its the original fob you need to de-activate the alarm system with a 4 digit code from the vehicles security card, I cant recall how to do it but details are in the owner's hand book. once the alarm is dissabled you need to hold the fob close to the vehicle and press either of the buttons on the fob 4 times or more in quick succession.

Unfortunately you don't.

It has to go to the main dealer or a suitably equipped automotive locksmith: reprogramming requires that the vehicle plus all keys/fobs which you want to use are presented for reprogramming together. If you forget to take along any existing fobs or chipped keys then they probably won't work afterwards.

Miss Barbie, I've been through this, and you must call a locksmith to reprogram your key to the vehicle key code. It's the only way Luv.

alarm needs to be reset to the key fob thats all,

if it is a mems unit , it will need to be programed to the key fob and alarm . (there is no way round it).