Moved to the recording / editing / production / software / hardware forum.
It's probably just your local browser that is displaying the old version to you from its cache (because the thumbnail has the same filename) It's probably showing the correct version to everyone else
To solve it for you: Press CTRL-F5 on your keyboard when viewing the thumbnail on YouTube and that will force your browser to download the latest version.
If that doesn't work, clear your browser's cache and try again.
If that still doesn't work, change the name of the thumbnail image on your local hard drive and upload it again. Since it's a different filename, your browser will treat is a different file and will try to grab the latest version.
hth