I have suggested that you could use antimatter to melt the core of Mars and restart the magnetic "dynamo", but it occurs to me that I never really considered exactly "how" you could do this. Melting the core of medium sized planets might actually be part of a far future tool kit to terraform worlds, so this is the sort of technology a space faring civilization would have.
Digging a shaft to the planet's core is out of the question, since the static pressure would collapse any conceivable tunnel long before you reach the core. So the question is "how would you go about melting the core of a planet". Antimatter is a convenient energy source, but I won't constrain any answers to just using antimatter, if you have a plausible, non magical means of melting the core of a Mars sized planet.
The Melting the Core question does seem similar, but most of the answers there either involve destructive heating of the entire planet or creating an artificial magnetic field without changing the composition of the core. I am looking at the idea as a way to solve several terraforming problems at once: generate a magnetic field, restart plate tectonics and the carbon cycle, recharge the atmosphere via volcanic outgassing and restarting the hydrological cycle as the crust warms up and the permafrost melts. The other difference is what technology or technique is needed to actually inject the energy to the core, something which is usually hand waved.