You can replace an existing version of a video using the top right button 'Replace...'
Upload the file from your local device
The new version will show as uploading at the top
Once the new version has been uploaded, it can take up to 30 minutes to be fully replaced.
All versions uploaded will be shown on the right side-bar where you can switch from one to another.
Use ‘Revert original’ to blast all revisions and revert to the original file version.
Find in your account settings the option to remove versions after a week of being added.