How do they change or insert the battery inside a pacemaker?

Change the generator: Small incision to access the generator box which has an integrated battery. Thus the whole generator is changed, not a separate battery, then the leads are reattached to the generator, placed back into surgically created pocket under skin, check function, close incision, good for another 7-10 years or so, depending on use.

They Don't: The battery is hermetically sealed (welded) into the titanium shell with the circuitry. We replace the whole device, only leaving the existing wires behind.