To refill a central heating boiler, you will need to:

1. Locate the filling loop, which is usually a pair of flexible hoses that are connected to the boiler. These hoses will have a tap or valve on each end.

2. Turn off the power supply to the boiler and let it cool down.

3. Turn off the taps or valves on the filling loop.

4. Attach a hose to the tap or valve on one end of the filling loop, and place the other end of the hose in a container of cold water.

5. Open the tap or valve on the other end of the filling loop.

6. Slowly open the tap or valve on the first end of the filling loop to allow water to flow into the boiler.

7. Keep an eye on the water level in the boiler and stop filling when it reaches the maximum level.

8. Close the tap or valve on the first end of the filling loop and remove the hose.

9. Close the tap or valve on the other end of the filling loop.

10. Turn the power supply back on and restart the boiler.

It is important to follow these steps carefully and to use caution when refilling a central heating boiler. If you are not comfortable performing this task or are unsure of how to do it, it is best to contact a qualified plumber for assistance.