Can anyone tell me how to reprogram a citroen xsara 2.0 vts key after changing the battery?

You do not mention if it is a mark 1 or 2 Xsara.

You should not need to reprogram the key as the battery is for the HF plip not the magnetically coded key.

To code the key you need to do so using Citroen's diagnostic tool (Lexia or Proxia)

To recode the plip:

Don’t press any plip buttons for at least 1 min

Insert key into ignition and within 10 sec press the locking button for 5 secs

Remove the key and do not press any buttons for at least 30 secs.

Coding these things can be a right pain sometimes, so i would try at least 3 times if it doesn't work first time.

And to the other answerer its a XANTIA not zantia!

and WHY should a xsara and xantia key coding procedure be the same?! the 2 cars use electrical layouts that are very different. although the earlier xsaras did have a cph personal like the xantia, but that quickly gave way to the BSI.