Hi
Have a look at this video after 7.50
https://youtu.be/1E3BKTSz9wABe aware that the battery is soldered. Removing it is quite simple, but soldering a new one may be dangerous. Instead, you can solder a battery holder in order to use standard battery.
I did that on mine.
Unfortunately, the holder is a little bit thicker than the original battery, so closing the box will be harder...
Important point : once the battery is removed, the factory settings will be lost.
On mine, this introduced an offset in the FM band (about 0.5MHz) —> not a very neat result 🙁