How can I fix it?

If this is a mechanized device and a coin is jammed, the mechanism is jammed. The may require disassembling the unit to reveal its internal mechanics to examine it up close. Whether gear or levers are used, sometimes its obvious where to reverse the mechanism to unstick the stuck coin. Good luck as this may result in damage as an inexpensive piggy bank. A rocket scientist isn’t needed as most parents have to deal with real world child raising issues more important than rocket science. Parents raise future rocket scientists.