Howtoforge published a tutorial about increasing the disk space of an AWS EC2 Ubuntu instance.

How to increase the disk space of an AWS EC2 Ubuntu instance

Sometimes there could be a need to increase the disk space of your servers. It is always recommended to take a snapshot of the volume before you extend it or increase the disk size. Also, it is not advisable to decrease the disk size. Increasing the size of a volume does not increase the size of the partition, its file system must be extended. One needs to use a file system-specific command to resize the file system of the volume. 

How to increase the disk space of an AWS EC2 Ubuntu instance