I allocated only 10 GB space for my Ubuntu 12.04, but now I want to add extra 30 GB for my OS because that 10 GB was not enough for my usages. Now what can I do?
Try GParted or Partion Magic
you can mount it.
Or, if you installed with Wubi, everything you need to know is here:
Or you can go the other way round and install EASEUS Partition Master 9.1.1
(Freeware) in your Windows from the download site http://www.filehippo.com/download_easeus_partition_master_home/ and then increase your Ubuntu installation partition graphically. I presume it will be a simpler method. :)
Well you will need to go into your your main partition (like windows) and open "Create and format hard disk partitions", then shrink the partition for Windows by 30GB, then boot into you Ubuntu INSTALL DISC and open Gparted then expand your Ubuntu partition.