I have a few solar powered radio/controlled Casios, and I have replaced the rechargeable battery in a few of them, the batteries are about £8 and it is easy and straightforward to do it. My EDC one is a WVA-430U I changed the battery about 8 years ago and it is still doing fine, an excellent cheap watch.
for batteries look here....https://www.watchbattery.co.uk/shop/products/WSCS-10152407.shtml