You can change either the glass alone, or the whole glass/digitiser assembly (which is easier and less likely to go wrong imho). The glass costs around 45$ US on ebay, the whole assembly around 150$. I changed mine last week. I used etradesupply.com who will sell you OEM or also copy parts if you want to take the risk. They were very fast, they're in China and I'm in Dubai. - no problem. Videos of how to do it are on youtube. You need a couple of very small torx screwdrivers only, if you change whole assembly.
Of course, if you do it, you take the risk/void the warranty. My 920 is more than 1 year old....
kr D,