Merge partitions on debian server

I have upgraded my VPS with more SSD (from 20GB to 40GB), but don't know how safely to merge the new available 20 GBs.

Here's the output of sudo fdisk -l:

sudo fdisk -l

Disk /dev/vda: 42.9 GB, 42949672960 bytes
4 heads, 32 sectors/track, 655360 cylinders, total 83886080 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000

   Device Boot      Start         End      Blocks   Id  System
/dev/vda1   *        2048    83886079    41942016   83  Linux

As you can see, there are >40GB in total. And here's the output of df -h:

Filesystem      Size  Used Avail Use% Mounted on
rootfs           20G   19G     0 100% /
udev             10M     0   10M   0% /dev
tmpfs           782M  140K  782M   1% /run
/dev/vda1        20G   19G     0 100% /
tmpfs           5.0M     0  5.0M   0% /run/lock
tmpfs           1.6G     0  1.6G   0% /run/shm
tmpfs           2.4G     0  2.4G   0% /tmp

What you I do? Thank you very much in advance!