The easiest way to submit data is to right-click on the map and choose "Report a problem". This pops up a small form with a marker. You drag the marker where the problem is, and you pick a category and type something to explain the problem. In a few days, a human being reads it and (assuming he/she agrees) fixes it.
For instance, Google Maps would send bicycle routes near my house to a road with a killer hill. There's a reasonable alternate route that adds only about a quarter of a mile, but avoids the killer hill for a much more gentle hill. So I reported it.
In a few days I got an e-mail that said "Your Google Maps problem report has been reviewed, and you were right! We'll update the map soon and email you when you can see the change."
Sometimes you'll also get an e-mail afterwards that says "Google Maps has been updated to correct the problem you reported. You can see the update here, and if you still see a problem, please tell us more about the issue: (link)"
And now those same bicycle routes, by default, route along the gentle hill instead of the killer hill.