Extend trip - visit japan web?

i have already been in Japan for about a week, as a tourist, from a no-visa-required country. I have originally planned an under 3 week trip but now I suddenly have more time and so want to stay a few more days (it will still be under 4 weeks).
Is this possible, and how do I manage VisitJapanWeb? Do I need to go somewhere and speak to someone (which would make it not worth it imo) or just a few clicks on the web? Even with the few days extension it will still be less than 4 weeks and I believe visa-free travel is up to 3 months so I am well within that time limit. However I am just wondering if I can edit my visitjapanweb now that I am already in Japan, and if it needs to be discussed with or approved by anyone?
If it can be easily done that will be great and I can spend a few extra days sightseeing, if it cannot be easily done then I will stick with my existing schedule.

Thank you.

You won't need to chamge anything. The visitjapan web page is just replacing the old immigration cards, those were also not able to be updated.
The most important part is the sticker (or information if you didn't get one) in you passport. It shows the validity of your stay and until when you have to leave. As long as you don't overstay it shouldn't matter