upgrading my electric bike battery. Li ion 48v 625wh battery
If you would be able to safely open up the battery pack, which I do not recommend, you could hopefully if labeled read out what battery cells there is within it.
From that you could hopefully find a battery with higher mAh, but here you would have to be sure that the same voltage levels is acceptable for charge and discharge and that the peak current is at least the same and that you have a temperature sensor on the package to monitor the heat. If the battery pack doesn't have and temperature sensor, the battery may have a higher internal resistance than the other one and not be designed for it and may overheat during charge and use.

However if you could find some batteries that fits in you could, replace them. but then you would have to be sure they are in the same batch and well balanced to avoid uneven charge and discharge circuits.