Yes, there are replacement batteries (see here on digitec), but I can't tell you how difficult it is to replace them, but it should be possible if you know how to use a screwdriver - soldering or something like that is not necessary, the battery is plugged in via a connector.
I can only recommend the Robotor in general, I am very satisfied with it. Exception: You have (many) pets, then the brush can be a bit annoying, but generally without pets or with a cat/dog you should have a lot of fun with it.