You do not HAVE to replace them. There are 2x CR17450E-R FDK 3v 2400mAh lithium built-in.
With a little skill you can get them replaced, but I think the batteries will most likely outlast the life of the product.
I would ask myself whether Netatmo is the right product. It took me a few attempts to get the setup working (including a warranty claim via Digitec). The Legrand support site and support itself is unusable.