You use lsblk and see your EBS volume is the right size, but running df -h shows the device is smaller. To fix this, the command to use is resize2fs
resize2fs /dev/xvdf
While you can do it on a mounted device, these things are often better done when it's unmounted, just to be safe.
https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Storage_Administration_Guide/ext4grow.html
