For individual files, you can upload new versions of the file by clicking the the V1 button in the top left of the file viewer and clicking Upload New Version.
You can also quickly upload new versions on the folder view by hovering over the file thumbnail and then clicking Upload New Version.
In the folder view, the current version of the file is displayed directly under the file name.
In the file view, the current version is displayed at the top left of the screen, and you can easily toggle between prior versions of the file to view the file and past comments.
Pro Tip: Use version control to track a file's changes over time within the file, rather than creating new versions of the file in the folder. For instance; if you created a file named "Script", once you are ready to add new versions of the script, simply upload them to the original "Script" file and track the version changes via the built-in version control.