Top of key shaft where it joins plastic/bottom corner of blue casing, there is a increase in the size of the casing join. Screwdriver/nailfile/ teeth in there and lever GENTLY apart.
Note the way the battery came out and put identical new one in the same way roud and clip together. Shouldn't need reprogramming.
However take care when prizing apart as it may not be a battery fault but a detatched micro-switch. If this is the case do a search in the 'search' box here for details of how to repair that.