You can resize the droplet via the Reszie menu in the Control Panel.
You can check this part of the docs:
You can resize Droplets from the control panel or using the DigitalOcean API.
Before you can resize a Droplet in the control panel, you need to power it off. We recommend you do this from the command line to avoid data corruption, so SSH to your Droplet and issue the shutdown command:
sudo shutdown -h now
Next, go to the DigitalOcean Control Panel. On the Droplets page, click on the name of the Droplet you want to resize, then click the Resize option in the Droplet-specific menu.
The current Droplet size is highlighted. Choose CPU and RAM only or Disk, CPU, and RAM resizing, then select the new Droplet size.
Once the Droplet is powered down and you’ve chosen its new plan, click Resize. A progress bar displays as the resize takes place.
When the resize event is finished, click the On/Off button to power the Droplet back on.
In certain cases, a disk resize fails to resize the Droplet’s partition or filesystem. If you rerun
df -h after a disk resize and the output is unchanged, this usually indicates a problem.
This can be sorted using resize2fs for ext3/4 filesystems