1)Turn off the boiler power switch so it cant turn on.
2)Connect the garden hose to the red valve (the one near the front of boiler in your picture).
3)Close the blue valve below that red one. (don't touch the other red and blue valves near the rear of boiler)
4)Open the red valve to let the water and air come out into a sink, or outside, or into a bucket.
5)While its draining lift the lever on the feed valve to allow city water pressure to help push more water and air thru the system.
When its coming out clean and no air anymore lower the lever on auto feed valve.
Close red valve to stop water flowing out hose
Open blue valve
Turn on boiler power switch and observe pressure as it heats up