Here's how to do it via the YouTube Using Enhancements page:
When signed in, click the arrow next to the Upload button at the top of any YouTube page
\Select Video Manager
Click the Edit button for the video you want to edit
Click Enhancements
Click the Slow-Motion Turtle and choose your speed.
Here I chose 50%.
When you’re done, click Save, or use Save as to create a new enhanced video, while keeping the original video untouched.
via Slomo makes everything better