Can you let me know how to change it and reprogram the fob if necessary.

Youtube!

With an original key (rather than a cheap replacement key shell), the key pulls apart lengthways, leaving you with two 'halves' - the bit with the flip key blade, and the bit with the buttons and pcb inside. The latter then splits in half again - you'll see a line running along the groove where the key blade lies when folded, prise the two halves apart. You'll see the CR2016 battery which can then be replaced. Reassembly is the reverse of dismantling. You shouldn't need to reprogram the fob, but if you do, proceed as follows - you MAY need to do this for each key that you have, even if they worked before;

1. Press and hold the unlock key on a remote
2. Insert key in ignition and turn to on (while still holding unlock)
3. Hold unlock button with ignition in the on position for 2 seconds
4. Turn key to off, remove from ignition and release button.

All you need to do is to find the openning. It's generally near the key ring, after that open the remote, pop up the battery put in a new one, but don't use a lot of force, because you can actually break the case. If after replacing the remote doesn't work, it means that the battery is upside down or the remote is broken. When it comes to reprograming a remote better not to do it yourself and call professional locksmiths. Actually I have used

azlocksmith.co.uk/mobile-locksmith-services/auto-l.../ services

and I'm very satisfied.