How can pacemakers be programmed?

Programmer. The piece of equipment used to interrogate and program pacemakers (and defibrillators) are called programmers. The programmer is used (usually) by a representative of the manufacturer. When devices are implanted they are usually set to 'norms' established for the patient's condition. Later, especially for bi-v, they are optimized. It is the cardiologist that determines the appropriate settings.
With a special box. Each manufacturer of pacemakers makes their own device that communicates with it by radio waves and can get reports from it and can give it instructions.