Can anyone help me to find the way to use a CD writer as a DVD Writer.

You can't. They are physically different pieces of hardware.

Buy an inexpensive DVD-Writer:
HP dvd1035i DVD Writer (Retail) - 48x CD-R, 32x CD-RW, 20x DVD±R, 8x DVD±R DL, 8x DVD=RW, 6x DVD-RW HPDVD1035I at TigerDirect.com (http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=3901538&sku=L12-1164)
Open computer (2-3 screws on back/side)
Unplug data cable and power cord from back of CD drive
Remove the 2-4 screws holding CD drive in place and discard
Insert DVD writer, reapply screws, data cable and power cord
Close the computer
Reboot, inserting the DVD-writer's install CD after system detects the new drive
Install the new drivers
Reboot again
Install additional software/utilities that came with new unit (off the CD) if desired
Enjoy new DVD writer