First install growpart:

yum install cloud-utils-growpart

Check current disk info:

fdisk -l

Disk /dev/vda: 42.9 GB, 42949672960 bytes, 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 label type: dos
Disk identifier: 0x0008d73a

   Device Boot      Start         End      Blocks   Id  System
/dev/vda1   *        2048    41943039    20970496   83  Linux

Check disk partition info:

df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/vda1        20G   15G  4.5G  77% /
devtmpfs        486M     0  486M   0% /dev
tmpfs           496M     0  496M   0% /dev/shm
tmpfs           496M  460K  496M   1% /run
tmpfs           496M     0  496M   0% /sys/fs/cgroup
tmpfs           100M     0  100M   0% /run/user/0

Run growpart on our device:

growpart /dev/vda 1
CHANGED: partition=1 start=2048 old: size=41940992 end=41943040 new: size=83883999,end=83886047

Resize:

resize2fs /dev/vda1
resize2fs 1.42.9 (28-Dec-2013)
Filesystem at /dev/vda1 is mounted on /; on-line resizing required
old_desc_blocks = 2, new_desc_blocks = 3
The filesystem on /dev/vda1 is now 10485499 blocks long.

Check if disk resized:

df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/vda1        40G   15G   24G  38% /
devtmpfs        486M     0  486M   0% /dev
tmpfs           496M     0  496M   0% /dev/shm
tmpfs           496M  460K  496M   1% /run
tmpfs           496M     0  496M   0% /sys/fs/cgroup
tmpfs           100M     0  100M   0% /run/user/0