You can easily add new versions to any of your files or designs from the Files section in any project. To add a new version, find your document in the list of files and click on the row to highlight it. When the sidebar appears, click the + button to the right of Versions.
Select from the list of options for uploading the file and then click Upload a new version or drag the file there.
Click Done and you will see the latest file just uploaded and the previous versions listed underneath it.
An email notification of the new version will automatically be sent out to everyone who has permissions to view the original file. This is to inform all users that a new version has been uploaded.