Please note the following to avoid damage to your grouphead:
From the top of your grouphead, the gicleur/injector can be removed usually with an 8mm socket and socket wrench. Upon installation, needle nose pliers can be used to start the installation of the new gicleur. Otherwise, place a little white masking tape on the inside of the 8mm socket to hold the gicleur in place.
WARNING!!!! Upon installation, only hand tighten the gicleur. If you use a socket wrench, there is a 99% likelihood that the gicleur/ injector will break inside the grouphead and can be difficult to remove.