Hey,

Being on Cloud VPS, its pretty dam simple but still remember, depending on your existing size of the disk, your website will face down time. Not much though, maximum of an hour or so.

Step One

Power off the droplet and then take a snapshot.

Step Two

Delete existing droplet.

Dont worry, the IP will be reserved for few hours or so.

Step Three

Deploy a new droplet using your snapshot (which you just took) with a larger plan.

Follow this below guide for detailed info and steps..

https://www.digitalocean.com/community/tutorials/how-to-migrate-droplets-using-snapshots