Machine off - Switch on the machine while holding down the "Menu" button, until all the leds are flashing on/off: it's the "maintenance" mode. - Press 80 - Press the "Mono start" button several times until you see "purge: xxxx" - Press "2783" in order to reset the purge counter. You will see: "purge:0000" - Press the "Stop" button. - Press "99" to exit the maintenance mode. This may or may not work but it's worth a shot.
I've just finished one of these, and it didn't go well. The way they've designed these things you have to use a decent amount of force to get things off and on--and it's not at all as easy or nice as a screen replacement on a Galaxy S3, for example. The new screen cracked the minute it got put on. I used the youtube video above and he seems to have skipped over a few little things, like making sure the little metal earpiece gets back in, etc. But it's a good general guide. You will definitely need a heat gun, or an iOpener.