Replacing the glass consists of, basically, heating the screen enough to loosen the adhesive but not enough to damage the digitizer. All it takes is a good heat gun (doing it with a hot plate is like changing a car tire on a wheel using large screwdrivers - technically known as "the hard way") and a lot of experience. Heat guns are cheap. Experience isn't - it costs phones. (Which is why a lot of shops keep "unrepairable" phones - to get experience in things like disassembling that model and removing the glass from the screen.
Doing it yourself with no prior experience usually leads to a few problems - like breaking the screen (digitizer, LCD and LED), losing tiny parts, breaking tiny parts, breaking not-so-tiny parts. The end result is usually that after you've spent about half the price of the phone trying to repair the things you broke, you search for a repair shop willing to even attempt to repair it for you (most won't) and end up paying a total of about what a new phone would have cost. Paying to have it done is almost always a lot cheaper (even though the glass and digitizer is only about $50).