It's not too hard actually. You unscrew the fuel pipe going from the main rail to the injector (careful, it can spray fuel, especially if you haven't let it all cool), unplug the electric cable that supplies the injector, unscrew a plate holding down the injectors (3 plates, each one holds 2 injectors, 1-2, 3-4, 5-6), pull the injector up and out, replace with new. If going with new it's much simpler and you need less special tools, because if replacing existing then you need to do a bit more work with the seal on the end of the injector which has to be replaced if you don't reinstall the injector within 5 minutes of taking it out (supposedly the sealing ring swells up).
I would do this on a cold car and I would unplug the battery just in case, it's never a bad idea IMO.
However, since your issue started after changing the plugs, it's probably not a coincidence!