Increasing the size of a volume, connected to your droplet, is easy with digitalocean.
Note that it is recommended to shut down your connected droplet before resizing a volume.
The only issue is that though you increased the size of your volume, the newly available space will not reflect into any of your droplets until you have changed the partition table. But this is as easy as running one simple command if your partition is ext4-based:
sudo resize2fs /dev/disk/by-id/scsi-MY_VOLUME