A cellphone's screen generally has two major parts, the actual LCD and the glass. The glass covers the LCD, providing protection against accidental damage and occasionally acting as a digitizer on touch screen phones. While the glass may be easy to replace on many phone models, if your phone has a crack on the LCD, repairing the device will require that you take it almost completely apart.
Remove the battery cover, battery and any cards, like SIM cards or SD cards, from the device.
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Remove the screws from the back of the device, along with the screws around the LCD, if your phone is a flip-style device. The type of screws will vary depending on the manufacturer and model of your phone, but will generally be either T4, T6 or #00 Phillips screws.
Pry the housing off the of the device using your pry stick. Once you are sure all the screws are out, just insert your pry stick anywhere between two piece of housing, then move the stick around the perimeter of the housing to disconnect the clips holding the pieces together. Continue to remove pieces of housing until you are left with just the control board and LCD, sitting in half of their respective housing.
Disconnect the control board from the LCD, and the digitizer, if necessary. On most devices, you will find a cable running from the LCD to the control board. Lift up on the connector to disconnect the cable. If you have a touchscreen device, you will have to perform the same task on the cable running from the glass to the control board.
Pry up the LCD to remove it from its housing. On some devices, you will also have to pry the glass off of the LCD. In these cases, using the heat gun on the glass for about 30 seconds will loosen the adhesive enough to separate the glass. Note that some devices, like the iPhone, has the glass and LCD permanently together. When you get your replacement LCD, look and see if the glass is included. If it is, you can skip trying to pry the glass and LCD apart.
Place the cracked LCD aside and put your replacement LCD in its place in the housing. Reassemble the device by simply reversing the steps you took to get the device apart.