It has been overheating so we replaced the thermostat. It was doing okay and now it is overheating again. A mechanic told us that it may have air pockets causing the overheating. How do you purge the system?

This link may help:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-3.html#post235973

Totally fill the radiator with coolant, allowing the upper radiator hose to fill, and fill the overflow reservoir to the FULL Line. After the engine is at normal operating temperature, squeeze the top radiator hose several times to purge any air.

Personally, I doubt that entrapped air is the problem. Phosphate scale may be blocking the passages in the radiator. It's possible the radiator needs replacing. Check the radiator for "cold spots," by using an IR thermometer gun. The problem is usually something pretty big and basic.