To resize, you'll need to power down the Droplet for a few minutes. I would recommend doing this from the CLI using:
Once the Droplet is offline, you'll click on the name of the Droplet you wish to resize from within the DigitalOcean Control Panel, click on Resize from the left side menu, and then choose either:
CPU and RAM Only
Disk, CPU, and RAM
The difference between the two is that when scaling CPU and RAM Only, you can scale back down to the previous Droplet size once the traffic burst is over.
If you choose Disk, CPU and RAM, you will not be able to scale back down -- only up.
Once the changes have been made, you'd then need to click on Power from the left side menu and power on the Droplet. The changes will take immediately and you'll be all set :-).