The injectors sit within a copper tube which is swedged into the cylinder head
at its lower tip. Over time, carbon accumulates between the injector tip and the bottom of the copper tube, and also dirt, rust and crime and corruption build up around the injector body and the tube. The object is to get the injector out of the tube without disturbing the copper tube in the cylinder head
Probably the best course of action here is to soak the area where the injector enters the tube with penetrating oil
for a day or two to try and loosen the rust. Then try to rotate the injector with a wrench to break it loose. If you have room, you could use a slide hammer to try to lift
the injector, or a ladies foot to lever from underneath. Be careful when levering that you don't put too much pressure on the head.