Bad news: you cannot just replace the digitiser because it is glued to the LCD. Replacing just the digitiser without the LCD is nearly impossible without expert tools (vacuum pump, UV glue and UV light). So if you want to repair the cracked screen you have to buy a replacement with the LCD.
You can lift the whole digitiser with the LCD using a guitar pick, some other prying tools and a heat gun:
-Pry the silver lining loose around the edges with the tools and set it aside
-Warm up the edges of the glass with a heat gun (stay away from the screen itself)
-stick the guitar pic between the glass and the body
-Twist and move a bit to the side, reheat, repeat.
Be very careful not to put the pick too much inwards (you can damage the LCD)
Be very careful not to cut the ribbons of the LCD and the digitzer with the pick.
It is a very precarious operation which I would not recommend for DIY, but if it is already broken and you don't have any warranty then you might give it a try.