how can I be able to restore only the contacts from previous backup and not the apps and other stuff.
You cannot do a selective restore. You can, however, resync your contacts from the computer or iCloud if you were syncing them there as you should.
A Restore from Backup is All or Nothing...
Why not Re-Sync your Contacts...
SYNCING with iTunes
See here > http://support.apple.com/kb/HT1386
From Here > http://www.apple.com/support/iphone/syncing/
thanks for ur support but I dont want to use the old apps and photos but contacts.
how to re-sync contacts only???
Connect the device to iTunes, select sync for the contacts. Make sure you have everything else you do not want to sync turned off. Of course, this means that you have synced the contacts to a supported application on the computer to start with.
Retrieving data ...