Change the size of Google Photos after it been uploaded

I've been uploaded photos to Google Photos with original size instead of high quality. Now, my Google storage (Drive) is full and I want to change the size of the already uploaded photos to the high quality option for unlimited photos backup.

How can I change the size of the photos after it already been uploaded?

Here is a screenshot with an explanation from Google:

Source of screenshot

Thanks.

You can do it from the Google Photos web app settings page.

1. Go to http://photos.google.com from your browser.
2. Click on the settings button ().
3. Click the Recover Storage button.

That it :)