1. Choose droplet and click menu “Resize”
2. Choose the new package. Click on “Turn off” droplet and Click on “Resize”
3. There is a progress bar just below the droplet name on top area to show progress of the resizing
4. Wait until resize successful message “Droplet has been resized” displayed.
5. Once the resize completed, go to menu “Power”. Turn on back the droplet.

The whole process may take 2-5 minutes. The estimation given is 1minute for ever 1GB disk size but normally it is a lot faster.

DO reference:
https://docs.digitalocean.com/products/droplets/how-to/resize/