The air vent sits atop an air scoop with expansion tank under. How do I replace the air vent? Each time I try water spurts out! ( Rookie ) Please help. Thanks

Shut off the water supply and drain down the boiler just a bit. Then you should be able to remove the old and install the new.

Naturally before you do any of this make sure the power is off for the boiler.

Couldn't you isolate it with the shut off valves