Basically do the following:
- Go to settings
- On the left, Taskbar
- Turn off the enhance option at the top
- Restart explorer
- On the left, Start Button
- At the bottom, set it back to "On"
The current enhancements to the taskbar (aka the Stardock taskbar rewrite) doesn't allow the Start button to be in the centre. Hopefully it will get added as additional option in a future release.