how to replace glass in computer ?

It depends on the brand and model.

You can...
1 - Actualy read your user's manual. There should be a trouble-shooting, maintenance and repair section that you need to consult. Should be available in printed form or in electronic form, normaly on the drivers and accessory CD that came with the printer. Also, normaly available on the maker's internet site as a PDF.

2 - Reply here with the relevent information: Brand and model. Preferably after checking your documentation.

The cost of the replacement part CAN be more costly than buying a new device.

Electro
January 2012